Maybe after you have customized the incident class using the Authoring Tool to extend the class and customize the form you want to hide the Extensions tab. How do you do it?
Simple – import this MP:
http://cid-17faa48294add53f.office.live.com/self.aspx/.Public/Examples/HideIncidentExtensionTab...
Before:
After:
Update (Sept. 16 2010): Note: If you customize a form, behind the scenes the Authoring Tool will create a new <Form> element in the MP. This new Form effectively "replaces" the out of the box form. If you want to hide the extension tab on the Incident form in this case you need to change the Target attribute of the Category to point at the new <Form> element's ID instead of the one in the MP example I provided above.