Note that your method will only affect existing Event lists (list type 106). If people create new lists after they will be able to create workspaces there.
If you have publishing enabled on those sites, you can remove the different workspaces templates from the list of allowed templates for subsites.
Alternatively perhaps something can be done with a feature stapler on the workspace templates.. Not sure what though :(
Edit: You can remove the Create Site right from a group of users, they will no longer be able to create workspaces (nor any other type of site...). Unfortunately they will still see the option and it will give them an error if they attempt it.
You can also try this method (it only hides the checkbox). They can still create workspaces from the site action menu.