It's not unusual in America for people to solicit colleagues at work for money for charities. Sometimes this takes the form of fund raisers (like Girl Scout cookies) and sometimes it takes the form of outright donations. The pressure to give is enough of a problem regardless of the context, but this becomes worse if you are an atheist being pressured to donate to a religious charity. There are risks for atheists who take a stand, but opposing this kind of behavior can be important and provide a basis for activism.
Standing Up For Your Rights as an Athiest
It is unfortunate and ironic that this would ever happen because the moral and charitable components of donations are undermined when there is pressure applied to the potential donor. Charity must be voluntary, otherwise it isn't really charity anymore. Sometimes, the pressure can be enough that it may feel like extortion with a pretty label applied to it.
You have a right not to be pressured by religious believers to donate funds to religious organizations for he purpose of fulfilling religious missions when you are simply trying to get work done. Unfortunately, when religion is involved, there are also a lot of strong emotions that can prevent simple and obvious solutions from being effective. In principle, you should be able to just voice your objections, point out that the solicitor probably wouldn't appreciate being put in the same position, and that would be the end of it. In practice, though, you may have to go further.
How to Respond to Religious Solicitation
First, be sure to document everything that happens: every solicitation and every conversation along with date and time. You'll also need to document every meeting you have on the matter: who attends, what they said, what you said, and what sorts of decisions were made. If you have to take this outside the company, all of this information will be critical when it comes to making and defending a legal case. Although it may be tempting to record conversations, it's probably illegal where you live so don't.
Second, familiarize yourself with all local, state, and federal laws dealing with religion in the workplace generally and your sort of workplace in particular (if applicable). You need to know what rights are protected and what isn't protected. Third, familiarize yourself with any company policies regarding religion and charitable solicitation. Smaller companies may not have anything in writing, but larger ones should and there is a good chances that those policies spell out what is permitted and what isn't.
Armed with all this information, your next step should be to talk to your manager and/or the Human Resources department. Be well-prepared with what you want to say, but make a point of not being confrontational or accusing. Even if others don't share the religion of the solicitor, they may be sympathetic to the idea of religious solicitation generally. You're task will be to help them understand what it's like on the other side of such behavior. You'll need to emphasize the fact that such solicitation is an inappropriate distraction which makes your job harder to do.
Standing Up for Others' Rights
What can make this a subject of activism rather than just getting some peace and quiet at work is standing up for others' rights as well as your own. This means looking for opportunities to go beyond merely ending a single inappropriate practice and taking proactive steps to educate people in your workplace about the limits of appropriate religious behavior at work, the rights of minorities, and the equality of nonbelievers. Your bosses may not be interested in this and there is probably no legal reason for them to allow it, but it's worth exploring because you could have a far more positive impact on people this way.
Assuming that your bosses are amenable, there are several things you can try. You could go with something as simple as drafting a letter explaining your position and the importance of treating atheists as equals. Not everyone will agree with it, but simply presenting your position to everyone in a clam, non-confrontational manner may cause a few to think differently about any assumptions they have had about atheists and/or the role of religion at work.
On the other end of the scale would be holding a workshop or seminar on religious liberty and equality in the workplace. It may be impossible to force everyone to attend, but a professional presentation would still probably have a more lasting and significant impact on people than a letter. A workshop may also do more to encourage productive dialogue and this is important because people who talk with one another are more likely to resolve differences and misunderstandings than people who keep avoiding difficult or uncomfortable subjects.
Is it Worth the Risk?
You will need to weigh the risks in all of this before you take matters very far. By aggressively promoting and pursuing your equality as a nonbeliever, you are sure to at least make some people feel uncomfortable and may even outrage those who are utterly convinced of their own superiority.
There is no shortage of true believers who are certain that Christianity and Christians should be privileged and some of them may even be managers where you work. They may have to accommodate you by ceasing high-pressure religious solicitation, but that doesn't mean hey will take it so well if you try to use this as a basis for further activism and education. You'll have to be prepared for the fallout.