Yeah, I've never encountered the person myself but several people have described instances of him/her being a dick in various ways. Probably best to do what pete said. I'd question whether someone who calls themselves assassinmaster has the mental faculties or social consideration to respond to a warning anyway.
Just putting this out there, assassinmaster00 !== assassinmaster00--
Could it be that the whitelist doesn't work correctly and is doing a in string (whatever the strstr equivalent is in Java) comparison?
Or it's tripping over the --. In (My)SQL -- is the start of a comment and if it's not escaped correctly it can cause problems. Someone here searched for 'assassinmaster00--' this morning and Sphinx tripped over it for instance.
Yeah, that was me looking for mention of them here.
I haven't seen them in game, but the signs they leave use the -- bit; I didn't know if their username did, but the beehive message I replied to didn't have them.
But if patch caught them wrecking my obsidian roof, it's probably is the same person.