You can't stop him, but if he continues to do this, the IRS will hopefully take notice. You need to arm yourself with as much proof ans you can find to support the fact that they live with you. See the IRS links below.
File your return as you normally would. You will have to mail in your return. You should get your refund, but they may ask you to submit these forms with the proof. They will have him pay back his refund.
https://www.eitc.irs.gov/eitc/files/downloads/f886-h-dep.pdf
https://www.eitc.irs.gov/eitc/files/downloads/f886-h-hoh.pdf
