Two ways to do it...
1 (the easy way): Open ActiveSync, select File->Delete Partnership. The PPC must be disconnected, and this only deletes the current partnership (the last one you synced with).
2 (the hard way): Open Regedit, go to HKEY_CURRENT_USER\Software\Microsoft\Windows CE Services\Partners -- this key will contain a subkey for each partnership. You can tell which PPC the partnership belongs to by looking at the strings in each subkey - one of them should be the name of the PPC. Delete the entire subkey for the partnership you want to delete (don't delete the FFFFFFFF subkey). (I'm not responsible if you fvck up your registry...)