Howdy,
Since this is the Pro forum I gather that you are not referring to the Community Events form. I can not think of a great way to do this. It would really involve some direct edits to the plugin, which is always a bad idea since they have to be redone each time you update the plugin.
For most of our users we find that they do not end up giving too many people access to the WP admin area. When you do give someone access they tend to require a little bit of training anyways, to make sure they do not mess something up. Perhaps mentioning that you don’t want new categories could be part of that training process?
Does that answer your question? Thanks!
– Brook