A pubstomper is a name for someone who usually goes in alone or with a group of people to a public server on a game and complete trashes them.
It often originates from games like Dota and Team Fortress where there is large variety in people's skill levels, some being amazing and some being downright awful.
Pubstomper: Hey manwanna go pubstomp some nubs?
Pubstomper 2: Sure!
Nubs: ermahgerd u faegit imma report u 2 gaben he wil ban u 4 lyfe u hakr!!!!111
(Scottish) Singular variation of the word pubes to give it a more demeaning effect when using it in an insult and creating more comedy value for those around you when you say it.