@Dale86 You either remove the washing machine from your amenities offers, or you make rules about washing machine use. Like 2 loads a week or whatever you think is reasonable for the number of people you host. Make sure to add the rule to your listing info so they can't say they didn't know.
I never realized that anyone would use the washing machine to wash one pair of socks and a pair of underwear until I read here about it happening to many hosts.