Are you wanting to block certain buffs from being casted on the merc or block certain buffs from mercs being casted on others?
I've never found a way to do it. So that I could get my caster merc to get my druid skin buff, instead of the cleric merc's HP buff, for example. Or to block the slow or promised heals for the caster merc, just like I block them on myself.
After we throw out buffs to the whole group I just go through and click off whatever buffs I don't want on the mercs.