I had a stubborn FAST Server installation that continued to return search results, even after the content source was removed from FAST!
After stumbling around, someone on the FAST forums at Microsoft suggested clearing the sp collection, but didn’t say how.
On the fast server, there will be a shortcut to launch a FAST powershell prompt – open that
Enter the command
That should clear it out – you’ll need to do full crawls on all your content sources after this is done to repopulate the index, so it’s best not to do this to a production box without understanding how long search will be down.
Also note that in FAST Search for Sharepoint, pretty much everything is stored in the SP collection – It’s my understanding that as of right now, you can only have one collection in FAST for SP.