When Was Pierce Banned from Greendale?

It was reported that he used a racial slur following a disagreement when filming the Season 4 episode "Advanced Documentary Filmmaking," and he walked off set. Shortly thereafter, it was confirmed that he would leave the series.

Why did Pierce get banned from Greendale?

Pierce is banned from campus for sexual harassment, but is still there in spirit (or rather, hologram form, telling people that the place is crappy, but still better than not being there).

Why did pierce die in community?

Troy decides to accept the offer, leaving the rest of the group, particularly Abed, in shock. During the credits, Mr. Stone has become intoxicated after joining the group at a bar. After rambling about his aspirations, he reveals that Pierce died of dehydration while collecting the sperm samples he gave to the group.