If they have a lot of HP, have your ranged champion(s) shred away at it with Kitae's Bloodrazor ( deals 2.5% of the target's maximum HP as magic damage -- basically, it's a weaker version of Madred's Bloodrazor. more info here: http://www.mobafire.com/league-of-legends/item/kitaes-bloodrazor-128 ). This should be purchased by a ranged champion with a high attack speed. Hit-and-run harass them until they're too low to defend (or until you can score a kill on them).
Kog'Maw would also be an effective counter, especially with Kitae's Bloodrazor, since his W skill is essentially a Bloodrazor in itself, and because his longer range (and great synergy with items like Frozen Mallet) will allow him to harass even harder. And it goes without saying, but his ultimate is another great way to chip away at / drive out those champions who like to camp at their turrets (also note that his ultimate is great for defending as well, since a single hit from far away will stop a capture).