Report slander??

Recently, someone I used to consider a friend said "knock knock." Of course i responded with "who's there?" He proceeded to say "you're mom gey." I took great offense to this, and I come from a great Christian household, so my mother could never even consider being a filthy heterosexual. Ew. Anyway, this person repeatedly contacted me via text message saying, "Lol! Stibarc is triple gey!" I know this is not true, but I am unsure how to report this slander to the authorities. Any help?