Need Help Preventing Sheep From Turning Grass Into Dirt.

Hi there everyone. I am currently faced with a problem on my server where there is sheep everywhere going around eating grass everywhere on the server and leaving dirt everywhere. It simply paranoids me and it leaves the landscape looking really ugly and "destroyed". Is there some kind of plugin that prevents the grass from turning into dirt? I was thinking about disabling sheep in general, but since we are in fact an SMP server, that'd be a bit impossible to do. I'd appreciate any suggestions that anyone can give me.