I always liked the Contingency; Heal tactic v. Harm abusers. If you really feel the need to alter the Harm spell, how 'bout making it higher level? Keep Heal at 6th but make Harm (as written) 8th or 9th level.
It's a damn nasty spell, no doubt, but it's such an obvious tactic that I think only DMs or players who are unprepared will get caught with their pants down, so to speak. By the way, all of those spells listed by Lucius
are valid anti-Harm tactics. True, they defend against the spell in an indirect manner but they are still defenses. Use fly, levitate, spider climb, and ranged attacks to decimate the annoying Cleric. Fighters can take advantage of anti-magic field to wade in & chop the poor sod to pieces. Repulsion & Bigby's are also great ways to neutralize any up-close threat. Once the Cleric is kept at bay, just pepper him w/ ranged attacks and spells 'till he drops. Rule of thumb; always take out the Spellcasters first. Clerics, Wizards/Sorcs, and Druids should be your primary targets in any major encounter.
Don't have a decent ranged combatant? Why the hell not!?
Seriously, every party should have at least one, preferably two combatants who are competant with ranged missile attacks. Even if it's not their primary means of damage-dealing it should be a reasonable fallback. There are times when wading in and chopping the enemy to bits just isn't feasable or tactically sound.