how do you keep ants from your acer

Cute kid! Chances are you've got aphid infestation and the ants are after the honeydew they excrete. Check out the plant thoroughly and look for colonies of blackfly/greenfly, whatever and when you find them, SPRAY! with insecticide, preferably systemic, that kills off aphids (which is nearly all the proprietary sprays out there)

Or you can spray with soapy water which is less harmful to the planet