I think Scaling Stopper might fix that. Are they always wearing eleven/glass or only sometimes? I guess it would make sense if thieves sometimes wore rarer armors.
I was looking around the CK and I think I found the list:
http://s1.directupload.net/file/u/16073/zo7jovg6_jpg.htm
So, as you can see, it is simply scaled by level. I changed the levled-lists by deleting elven, elven-guilded and glass from the armor, boots and gloves lists. I will test it - I might still have a savegame shortly before encountering a roadside thief - and report back.
And if there's really no simple mod like this I will upload this to the Nexus myself.
HEUREKA! It works.
http://s1.directupload.net/images/user/120620/qysajnqj.jpg
http://s7.directupload.net/images/user/120620/9w2q9g8m.jpg
Going to test a bit more, then upload the mini-mod to the Nexus. Someone else might also appreciate it. ^^