I think you'd be better off trying to block the jointeam command sent from the client rather than trying to block some random thing that has "team" in the name since there is a high likelihood that you'll mess with some larger more complex operation.
EDIT:
I just tested registering "jointeam" with register_clcmd() and then returning PLUGIN_HANDLED if I want to block joining teams and PLUGIN_CONTINUE if I don't want to block it. I used a global variable to control which return value was used and toggles that value with a chat command for testing.