Hi Lafate,
Thanks for reaching out! Let me help you with that.
You could try a plugin like Bulk Delete to remove lots of posts at once. I did a quick test, it worked for me.
Once installed and you are on its page, then look for the section By custom taxonomy. There you should be able to choose tribe_events and then Event Categories, then simply choose the needed category. I also reco to restrict the plugin to delete the first 1000 posts, in case you are experiencing a script timeout.
Let me know if this helps you with your question.
Cheers,
Andras