switching from breastfeeding to formula

The past few weeks I'm having to give my 11 week old son formula on top of feeding off both breasts just to fill him up. He use to get full from one boob and now both can't even give him what he needs so I've decided to switch to just formula, but how do I get my breast to stop producing milk? they are so heavy and feel bruised I'm in so much pain what do I do