Do you have a run to confine them to? If so, now's the time to use it. If not, it may be time to consider building one...
Chickens are incredible creatures of instinct and habit. Their instincts tell them to roost as high as possible, and once they start they have to be re-trained. You'll need to get them into the coop a good few hours before they usually roost and lock them in. Getting them in may be the tricky part, that's where the run comes in handy! Once you've got them sleeping in the roost for a few weeks, you might try letting them lose again, but no guarantee they won't go back to the trees. I had a trio roost in a tree a good 2+ years with no problems, in all weather. Only way I got them in the coop eventually was building a run and locking all the birds up for good.