'Can I offer you a Bible?' or 'Can I pray for you?' should be met with the same responses as the 'Can I offer you a cigarette or a 'Coke' or 'Will you go out with me?' If the offers continue, so can the refusals or acceptances, until one of the parties makes it clear that the offer is no longer available or acceptable, IMHO. If one is not 'free' to inquire or offer; then, one is not truly 'free'.
Amen
Then live with the fact that no one is "truly free" in the work place. There is no free speech in the work place, if one wants to continue employment there. If you want to proselytize at work, and the boss doesn't like it, go get another job. The First Amendment applies only to Congress passing laws--not employment conditions.