You'd want to use Group Policies in a Domain if you didn't want them to apply to yourself on that computer and just the user. But if you don't have a domain, you could disable the run and still browse to your gpedit.msc it's in c:\windows\system32 if I'm not mistaken.
You wouldn't be able to just quickly change the wallpaper or run line until you disabled the policy again, that's the whole idea. :)
However...
If you know the user, and can get them to login (or use a login script to check if %username% = victim) then you could apply this registry key (and others that do similar things like disable wallpaper changing)
Disable the run line (without policy)
http://www.pctools.com/guides/registry/detail/151/
By changing the key in HKLU (local user) and not HKLM (local machine) you are hitting just your user and nobody else.
Again, using a domain and full featured GPO is much cleaner, but if you are in some workgroup setting than a reg key might be a good idea until they realize they can login as another user... :)