Most people troll because it's funny. Others troll out of protest or frustration, such as tanks or healers getting sick of bad players, so they sabotage them out of spite. It's not something you can just pinpoint to the source and say "ok let's cure the problem". I report trolls in duties all the time and I usually had a GM get back to me for more details in the report.

Also, people can't rejoin the same party if they got kicked out of the duty. That was patched in several months ago, so no worries about trolls rejoining.