Do nothing. The ants have to live somewhere and it's unlikely to be affecting the rose. They'll probably live somewhere else next year. I find that they like to nest in posts that has lots of loose fresh compost in it. By next year the roots of the rose will hopefully have filled out more so it will be a less suitable spot for an ants nest. They are very industrious and entertaining creatures to watch, so why not observe and enjoy!

Not quite sure about this though - I had an ant's nest under a rosemary bush in the ground and it killed it as it was taking up all the space where the roots wanted to be.
If the rose looks healthy I'd just keep sweeping up the compost and putting it back in the pot for now and then when the plant goes dormant repot it. Watch out for eggs in the compost yu remove.Next year stand it on pot feet or little stones or something to hold it clear of the ground and the and the ants will be unlikely to get into it.