Perhaps a tab control along the top when picking an object for events?
I think allowing them to be put in to groups is a good idea. However, the search box at the top of the Event Wizard was added for this precise reason. If youre objects are handily named you should be able to filter them out in a second. I still think groups should be added though for huge games.
Other places like object parameters though can get very messy.. I'm not sure of an ideal solution there because space is limited.