Find problematic MySQL users and databases
I helped a company find which databases were accounting for the greatest load. We collected slow-query logs, and then ran a report with pt-query-digest. Except we subtotaled by db, instead of by query fingerprint.

$ pt-query-digest --group-by db mysql-slow.log


There are lots of other options in this tool, so you could filter or sort on the total IOPS, or total number of temp tables, or total duration of queries, etc.

Once you identify the worst db, move it off your server until its owner learns to write more reasonable SQL queries.