One of the perils of Second Life is getting griefed which can take a number of forms. Last weekend I went hiking in the Ozarks-lots of pictures but no Second Life. When I returned to SL, I had a message from the owner of the sim adjacent to mine that someone had griefed my land and would I please change my options so strangers can't build on my land.
I am usually really careful about building permissions but had just bought two 512m squared parcels to add to my public land holding and had forgotten to set the land permissions. So my poor neighbors were confronted with this wonderful mess.
I guess griefers attract each other and apparently things were actually much worse and a bit explicit. Fortunately my neighbor was able to file an abuse report with Linden Lab against the griefers, and apparently the griefers were immediately banned. I get the impression that use of their particular ISP address was also banned. Not too serious a hassle for them but maybe enough so that they will think a bit before griefing.
All I had to do then is set my access permissions, which I immediately did. Just in case someone hasn't done this here is what to do. Click on your land to access the about land dialogue. Open the options page and unclick the anyone can build option-unless of course you are running a sand box or like to be griefed.