You must go in and manually delete enough unused items to get below the 7500 limit.
When I refer to 'items', I'm referring to rules, variables, policies, groups, hosts, etc...
Go to the search page and search for It will tell you the number of results and that is the number of 'items' you have.
Some are applicable only to a certain OS (Solaris, Linux or Windows) and you can modify your search to find just those.
I suggest deleting items for OSs you do not have (like Linux or Solaris) for starters.
Once get below 7500 items, you'll be able to generate rules.
Check out this thread:
http://forum.cisco.com/eforum/servlet/NetProf?page=netprof&CommCmd=MB%3Fcmd%3Dpass_through%26location%3Doutline%40%5E1%40.2cbfbab4
Tom