I had a great SL Halloween! My partner Cindi and I went to a Haunted Mansion. They had dancing skeletons, a skeleton rock band (The Rolling Bones), seances, ghosts, and lots more. There were bar stools that floated into the air when you sat on them, and one chair I sat in removed my head and floated it about five feet above my neck.
But best of all, my costume won me a prize! Tradewinds Yacht Club had a photo contest, and my pictures of a ghost girl haunting a ghostly shipwreck won me "Best Composition". Whee!
If you rez the free gift, it creates another copy of the griefing object, which moves itself up to a high altitude and begins making offers of its "free gift" to everyone in the region. You must then find it and delete it.
It is very small, and transparent. Fortunately, your local chat log will note its coordinates when it makes its offer. Go to those coordinates. Make transparent objects visible with CTRL+ALT+T. Also, go to the World menu and select Show More/Beacons. Turn on beacons for scripted objects in the Beacons window.
Now look for a tiny red cube with a set of crosshairs through it and a bounding box around it. That's the nasty little culprit.
If it makes another offer, it will also change its altitude. Go to the new coordinates and chase it down. Right click it and Delete it.
Don't bother Abuse Reporting the object owner if you run across one of these. They are probably an innocent victim just like you. But, like ExDepart, it may be worthwhile to Abuse Report the object's creator. (Sorry, I can't tell you who it is. I deleted the one copy I encountered before noting the creator's name.)