ok, so i was here just to know if i can make a guy my best friend and afterwards marry him, and i kinda have my answer in hands, so i think is just fair to give y'all (i don't know if you are still with this same problem, so...) a answer: there's a cheat called 'have multiple bffs (updated)' on mod the sims. i think - THINK! - i can post the link right below, so...
modthesims . info /d/539718 (no spaces)
if you don't like downloading stuff or/and don't think this mod is what you want - read the instructions first, i recommend-, i suppose the only known way is to drop down the relationship and declare enemy, maybe? (or just drop down a bit the relationship, uhu. ignore him when his at your door, or calling you... yeah, be a bitch).
well, i hope this helps in any way possible ._. you're welcome <3