How to deter drug-users from sleeping on deck of public playground. (a gentle solution preferred!?

there is a public playground that has become a home for vagrants, homeless and drug-users. as much as i would like to find a safe place for them...a children's playground is not acceptable. they bunk out on the metal decking, under a roofed area. i am thinking some gentle nubs or something...but it can't be a trip hazard to kids, either.