A person who takes great joy in being a royal pain in your ass for a reason of any sort.
More at: hilarious, schadenfreude
Capitalize and add exclamation point (multiple accepted) for emphasis.
The apostrophe should be mandatory but will be optional and used for special scenarios when the magnitude is too great for a simple capitalization.
Qualifying phrases ("You're such a..." or "You big...") are absolutely unnecessary for this one-word retort. Spacing is also key. Jer'k ALWAYS needs its own line. It's a loner.
When I login to instant messenger and log out in under 4 seconds after dropping a bomb on someone before they can respond, a typical response might be:
I hate when you do that!