If they are Ants then they are likely to be farming Greenfly and not attacking the plant itself. if you use soapy water (Fairy Liquid is good) then wash off any Greenfly then the Ants should also go.

Or use any insecticide which treats for aphids - you needn't worry about the ants (if they are ants and not blackfly), they don't harm plants at all, and what Kildermorie says about ants on plants is entirely accurate.