The English language has a great word in 'loitering.' It conveys so much - a purposelessness, a youthful or immature demographic, a body posture.
In a nearby town (Piikkiö), I have seen two signs conveying the same idea, but using different words. I think there is a verb for 'loiter' in Finnish, but that's not how this sign chooses to forbid that activity. Instead, it's: "Asiaton oleskelu (vesilaitoksen) aluella kielletty."
This is up by the water tower where we were orienteering, but I've seen it by a päiväkoti as well. Basically, it means "Being in the area without a purpose is forbidden."
Or: asia (thing) -ton (without) oleskelu (being) vesilaitoksen (waterworks') aluella (in the area) kielletty (forbidden). Well said!