I found a few classic posts indicating that disease dots should neuter mob regen:
https://everquest.allakhazam.com/db/...m9820935161310,
https://everquest.allakhazam.com/db/...00100892751713,
https://everquest.allakhazam.com/db/...#m999815671652 and
https://everquest.allakhazam.com/db/...99253902825801
The gist is Cazel had crazy regen unless a disease dot was put on him, in which case he became much more easy to deal with.
It is my recollection that DoTs also blocked (but didn't overwrite the buffs) regen - surely everyone remembers the frustration that was Rabies?
Currently you can't block mob regen using disease dots on blue (tested in Kael where the frosty berserkers regen 2% hp per tick, which was not blocked by disease cloud).
This seems a good starting point to accrue evidence for and against disease dots preventing regen for players and NPCs.