Advice on particular tenant hogging laundry facility

[Stephen Chaney]

I have a 6 unit property with a total of 8 individuals living in the property. ONE 2 person family is consistently hogging the laundry facility. There is only one washer and dryer for the entire complex because the layout doesn't allow for any more. I have received several complaints. I sent a general letter out addressing all of the tenants explaining to them the importance of not leaving laundry sit in the laundry facility. Still they keep doing it. I have posted letters above the washer and dryer and they still keep doing it. What should my next step be? Without me actually witnessing the issue I'm not sure I can come to them directly and start pointing fingers.

I could have a washer and dryer installed in their unit and up their rent. Since they are month to month. I'd prefer that to be my very last step though. Any advice would be appreciated.

Stephen