Facing duplex owner does not screen tenants & our tenants suffer.

We have owned this duplex for 30 years have been lucky to have good tenants that pay on time & keep the place up. We get high market rent because the interiors are very nice & we have a reputation for caring. Our problem is the owner of the facing duplex is a slumlord & all he can see is the cash. He does not screen tenants & gets the low hanging fruit. Purple bedsheets for curtains, clothes scattered about the yard, endless visitors on bikes or cars that stay less than 5 minutes or several that just hangout. Needless to say this makes a more civilized tenant uncomfortable & not wanting to continue our relationship. I have offered to buy out the owner, but I am sure the place is paid for & he has no incentive to sell. For him any cash is found money. I also considered a fence down the middle between the buildings, but I do not think that is the cure. Our latest tactic was to call code enforcement for the numerous code violations, but that is temporary not a permanent fix either. Any suggestions? Thanks!