In this link, there is a section on ant elimination.
First Rays' Home Remedies
Hope it helps. I didn't think ants really ate orchids.... I just thought that they were, for the most part, "indicator" pests (i.e. they indicate the fact that some other insect is there... Usually b/c the ant brought the pest onto the plant themselves - they "farm" other insects like aphids & mealies... etc). But, I could be wrong, it could be the ants themselves are eating your plant... Either way, you'll wanna get rid of them!
Good Luck!