Ants are NOT BENEFICIAL to orchids. They track bacterial and fungal infections to the plants on their dirty little feet and they are very efficient little farmers of pests which do harm to orchids.
The best way I have found to eliminate them is fairly simple. Buy an insecticide powder specific to black and red fire ants (usually will get sow and pill bugs too). Sprinkle some on a piece of newspaper outside, sprinkle some in the top of the plant, bounce the plant on the paper and walk away. Return in 15 minutes and repeat the bounce. Do this several more times and then leave the plant on the paper overnight. On the next day make one more bounce and you will find the ants should be gone.
Return the plant to its bench, wrap up the newspaper and dispose of it. I leave the powder on the top of the plant until the next watering when it gets washed in. Causes no problem on any genera in my experience.
In addition, you might want to find where the little critters are coming in and powder the trail.
CL