Also, a ban of even a single hour's duration can be effective.
I would include a (standard) note in Recent Changes, like:
"Please do not use this page for graffiti. Try the [[sandbox]] instead."
or
"Yes, you can delete an entire page. But please don't"
If they ignore the first warning, you could give them a second, final warning:
"Please do not make any more edits until you read [[...some policy page]]. Any more (graffiti/deletions) and we'll have to ban you."
Actually, I think we should come up with some standard (boilerplate) text for warnings & explanations to newbies/vandals. I would phrase the first warning so it doesn't scare away an innocent newbie, while giving the second one an air of finality (knock it off, or you're out of here).
And I second the idea of a self-expiring ban. (I'm tempted to unilaterally un-block any IP address that was blocked more than 7 days ago. I just might do so - except dear old Helga, unless I hear a strong objection.)
Ed Poor