This topic was discussed before in this thread
http://forum.tixati.com/support/174/ and there is some good information there if you're curious.
I find it ridiculous that private trackers are banning Tixati because it has a feature that allows it to identify itself as a different client. If Tixati were allowed in the first place, it would be irrelevant that you could imitate other clients. That feature is useful to get around the arbitrary bans that some private trackers have put on Tixati, but banning it for that reason alone is either a hilarious bit of circular logic, or sheer spite.
Any private tracker whose admins lack the critical thinking skills to see why that is a stupid reason to ban something is probably not a community you want to be apart of. However, this sentiment seems to be common and getting onto a good private tracker is not the easiest thing to do, so there might not be much you can do about it. Fortunately you can just use the peer-id spoof feature to get around most private tracker bans if you really want to use Tixati.