Tenant keeps putting cooking grease down the drain

[Judd Campbell]

I have a 4-unit building built in the late 70's.  All the units have hispanic families in them.  over the last year I have had to have a plumber unclog the kitchen sink for 3 of the families.  One guy texted me just last week and mentioned that his sink is backing up again. 3x in the last year.  The plumber has told me it was cooking grease in the lines, he spins the snake quickly when he feels the grease then runs hot water down the drain, that seems to open it up for a few months.  I have told the tenants that they need to only put water down the sink but I can see lots of cooking pans full of grease sitting in and around the sinks when I arrive.  I have informed the tenant that per their lease they are responsible for drain clogs.  he said he will not fix it and he is not going to be responsible for an overflow since he told me about it.  He has been in this unit for 3 years and his parents are next door (have been there for 7 years)  They have gardens planted out back and they have no desire to move, I would like to keep them but I can't keep paying $169 to fix the sink drains.  I have told him to put boiling water and dawn soap down the pipes but I have not heard back.  

Thoughts?