No way, if it is to be enforced on the server level. Here are 5 reasons why
1. Evasion of bans is easy (unless an admin catches you evading)
2. Any sane person who knows about the feature (I'm quite sure it is enabled by default nowadays) is using cl_guidServerUniq 1, which means that the GUID of a ban from one server, is completely and utterly useless for another server.
3. I would not trust bans from most servers (votes on a webpage is a decent suggestion, but I think its too much work for too little, for other reasons given).
4. Due to #1, most bans are very short, so it isn't worth it.
5. The tremulous community is small, and thus has a small number of griefers. Its not worth it.
On the other hand, enforcing it on the master server level is generally useless, as you can store the IP addresses of most servers to connect directly, and there is
an alternate master serverEDIT: Also, aren't you banned from the default master server?