Hello,
Sadly, there is no way that I know of for blocking a spell on a Merc.
So in theory, if you have a group with 2 real players (ie dps castors), one cleric merc and a tank merc, there could be issues where the cleric merc casts Complete Heal on the tank merc. This despite the rest of the non merc group members and any pets having blocked the same aformentioned spells.
Edit : If this is what you are experienced then I hope there is a way and I just dont know it yet. Did a search but found nothing on the net or forums so far..
Now I'm not absolutely sure if this is fact as I only use cleric mercs in my set ups. But, when using a cleric merc in a group or with my 3 box set ups (with no tank merc in group), I rarely if ever have a situation where that happens as the cleric merc almost never gets agro first and/or alone. As a player tank will usually take initial aggro and other group players- melee and/or dps castors- would usually stay ahead of the merc on the mobs agro list.
Unless something went horribly wrong like an overpull to camp, wipe situation or where the players drop aggro leaving the cleric merc to fend for his/herself etc.
Hope it works out for you.
Edited, Aug 12th 2013 11:00pm by hexeez