To be ignored, cut off, brushed off, blown off, shoved aside, and otherwise snubbed.
'Mabbed' is a reference to the character of "Queen Mab" in the made for TV series, "Merlin". Upon being ignored and forgotten, Queen Mab lost all of her power, and ceased to exist.
In short, she was a troll. What happens to a troll when you ignore them and don't give them the reaction they want, or the time of day? They get bored, and go elsewhere. Ignore a troll, and they lose their power. Forget them, and they become as useful as a corpse.
Keep bugging me and you're going to find yourself Mabbed. Now stay da fuck outta my Journal!