I would normally say boiling water but you don't want to damage the plant - all I can suggest is ant powder - or take the plant out of the pot, shake off as many of the ants as you can, and repot it. If you do that, make sure to get rid of any eggs which are lurking in the roots, or you'll get even more in weeks to come.
I did what Boxtops suggests on a patio rose bush last year, there were millions of ants and eggs! Just emptied it onto the grass and washed it with the hosepipe (of course you wont be able to do that!!) and repotted it. It has turned out lovely this year with many new buds and flowers!
I have read where diatameous earth is a good solution. Google it and it brings up all sorts of info. I would put the internet site here, but I don't know how to technically do it.