<p>I have an Events list in sharepoint and need to disallow users from having the ability to create meeting workspaces in the new event form.  Shy of customizing the new event form (which breaks attachment support), how can this be done?</p>

By default, in order for users to create a meeting workspace, they will need to be an administrator or Site Owner (specifically they will need the Create Sites permission).  If you don't give them this permission, they won't be able to create a meeting workspace.
This will disallow the user to create any site under the site where these permissions are set.  I'm not aware of a way to restrict access to a specific site definition but still allow users to create a different one.