There is no other way to kill the mobile, as far as I know. But have no fear, you can script around this. POL 097 introducted repsys hook: you can eg set a cprop onto character before attacking, and in hook, decide it does not affect its reputation. Example of this exists in 097 distro, take a look.
Another way is to use SetScriptController (
http://docs.polserver.com/pol097/singlefunc.php?funcname=SetScriptController&xmlfile=uoem.xml): unset it's value before applying damage, and reset its value after.