thats fucked!!!!
always try to avoid violence but don't let this guy tweak you. tell him he's better of sucking dick for nickels thengettin 50 from you. if that don't work,
just get fuckin mean, crazy mad dog mean. if he shows up again at your door don't say a word just break his nose. have something just inside your door (ie: bat, pvc pipe etc.) if he hauls out a weapon, destroy his life!!!

you've got two things going for you:
1) this assholes on probation so if he does you any harm at all (even threats), he's in jail with no bail.
2) he's on your property, and where i come from that means alot. If he tried to come in your house it's your legal right to fuck him up.

the best plan for knives is to level him with a two foot power kick. your sneakers will help prevent you from getting sliced, that or throw something at his eyes and quickly (and i mean quick!!!) punch the shit out of his face.