If you installed them from Flash Builder you just need to go to the common extensions folder and delete folder that matches your extension ID.
All extension are copied to a common location, the extensions folder in the CS Service Manager (CS5ServiceManager or CS5.5ServiceManager). This is found at:
In Windows XP:
C:\Documents and Settings\<user>\Application Data\Adobe\
In Windows Vista:
In Mac OS:
If you installed them via Extension Manager you can delete them from Extension Manager by selecting the extension and clicking Remove.
Hope this helps.