How long did you soak? You might consider putting bait traps on top of the medium?
This might help:
Quoted from AOS website: "Ants
Ants seldom cause any direct damage to orchids, rather they indicate the possibility of infestation by scale, mealybugs or aphids. To control ants it is usually adequate to remove the source of food that is attracting them so you must examine your plants and control the pests that attract ants. Some orchids,
especially cymbidiums, produce sugary secretions on the flower spikes. In this case, the ants can be prevented from reaching the plants by setting the legs of the bench into tins partly filled with oil or soapy water, to make an ant-proof moat."
Here's a past thread from this board about how to get rid of ants:
http://www.orchidboard.com/community...-rid-ants.html
There's some good information you could use in the future in this pdf about Pests and Diseases by Sue Bottom as well:
http://www.staugorchidsociety.org/PD...ySueBottom.pdf
Nothing about ants specifically though. :/
Sorry I can't be of more help! Good luck!