I hope that Microsoft will add this functionality in ActiveSync 3.8.
http://www.cewindows.net/applications/undocumentedactivesync.htm
The following key will force devices to connect as a guest instead of prompting you for a partnership:
REGEDIT4[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows CE Services]"GuestOnly"=dword:00000001
If you need to set it back for some reason use this key:
REGEDIT4[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows CE Services]"GuestOnly"=dword:00000000