The ants go on fruit trees and other trees after the sweet stuff that greenfly excrete. You can't do much about it. What I find annoying is that they walk up your arms and tickle and sometimes bite when I am pruning or removing diseased apples/pears. They do the tree no harm.

Sometimes ground up dried orange peel or fresh spearmint leaves repel them. If that doesn't work you may have to use a pyrethroid dust, but I would consider that a last resort.

Treat the trees with a mild pesticide spray suitable for edible crops to kill off the aphids - the ants will disappear on their own once they're gone.