It feels kind of like a public forum, but it really, really isn't. Facebook owns it, and has complete say over how you use it. It's "private property", despite being open to the public --- there are very few public spaces online (probably because it's really hard to monetize public space). To a lesser extent, the congressman owns his FB page. You're a guest. You shouldn't let yourself be fooled into thinking you have any kind of rights on Facebook.

It's obviously a kinda shitty thing for the congressman to do (unless you were being really disruptive in the comments somehow), but other than getting the attention of a bunch ofhis other constituents I don't see what you can do to make him think it's worth listening to you.