You can only be banned by way of the server console. This means that direct access to the server machine is required. If you were banned, it was done by the host, or someone acting on their behalf as server admin.
Now, consider the fact that almost all of the FL servers are made possible by individuals. For every server out there, there is at least one person making a sacrifice to make it possible. Said sacrifice could be in the way of money (game hosting service), bandwidth, or sacrificing a computer. In return for this sacrifice, they get to run their server however they see fit. Seems quite fair to me, nothing illegal about it.
If you have an issue with being banned, why post it here? Posting a complaint here is basically pointless, considering there is no way to know if the host even reads this forum. You've got a much better chance of working it out by directly contacting the host or admin of that specific server. If the server you invested all this time on doesen't offer contact information, it's time to move on to another. I'd advise choosing one that does offer contact information, just in case something like this happens again.
Fixitman
Admin: Fixitman's Universe
Server Website (a work in progress)