I can sympathize with you but the result is usually the same for everyone. Years ago, on Fel, the guild next door used to attack us through walls, speed hack, and attack us while we were invis and our complaints came to nothing. Eventually, they were all banned but not because of anything we reported but because they dyed stuff an illegal color; go figure.
Another time we reported a char with insta kill fireballs whom we followed and attacked on quite a few occasions, to no avail. To their credit UO did have one staff member (Big Tiny) who showed an interest to catch this guy. However, as you might guess he never showed when UO staff were with us and he eventually disappeared.
We did win an occasional encounter though, like filling up their house with dire wolves (when they really were dire wolves and not just the big dogs they are now) so that when they logged on they lasted around three seconds. No doubt harrassment on our part but it was fun to watch.
Those were the days before Tram, and every weekend was a battle against the local reds or thieves guilds. We weren't particularly powerful but some of the battles were exhilarating and lasted ages, and we did manage to kill a red or two.
Usually, those who use hacks are neither very good nor very bright, and when they are dead don't move so fast.
By the time a GM gets a message such chars are usually long gone, best to team up with a friend(s) and plan ways to sort this char out.
