buy your new "roomie" a $5 bag of really shitty schwag.....and when you're already smoking and he asks for some or starts for your stash and bong/pipe/whatev, be like, "Hey there, little buddy! I got you your very bag to say welcome to the pad. Only smoke your weed and never mine unless I offer and things are gonna be just fine!"
there is nothing I hate more than doing my own thing and smokin' up alone, and the moochy roommate comes in the room and reaches their hand out for a puff...... Uh uh, nope........