I don't think ants can kill a clematis per se, but if you have sandy soil, they can probably excavate around the roots to the point that they aren't in contact with the ground, and thus could die. Keeping the area full of organic matter and well-watered will deter ants forming their colonies in that area.

"the soil practically boiled with big black ants" ...

mmmmm, if you have that MANY ants, then it almost sounds like a nest might possibly be directly below your plant and be causing the problem. I have seen small ant colonies develop within clumps of perennials and indeed damage is done if the ants are allowed to do their thing.

Terry

Disturbing the soil around the roots is a good way for a plant to die. Air pockets around the roots are not good things. I would be out there with some ant killing granules in a heart beat if it were my clematis.

Oh dear, it sounds like they excavated the roots. Last year they were very heavy in that area, maybe that's why the plant has never done very good. The annuals I planted out there did fine though. I will have to try to figure out something so we can share the earth. I really don't want to kill them just because I want to put a plant where there home is. I don't think that is fair. I guess I will leave that spot bare and plant elsewhere.

Thanks for the answers. I appreciate it.