The civil servants and bank employees that process and read the checks, have nothing to do with passing laws, or with this guys grievance.
You want to write a "fuck you" letter to the city councilman who passed the law, the landlord who screwed you on your lease, or your ex-boyfriend. That's fine.
It sounds like the civil servants over reacted to me. But, I'm not willing to unequivocably state that you have a first amendment right, to write belligerent notes to civil servants and bank employees that are complete strangers to you, and who have nothing to do with your greivance.
It's not as though he addressed it to anyone is particular. It seems clear that his speech was directed at the authority itself and not to an individual. At most it was a generalized grievance to no one in particular. That seems as though it should be protected to me.