I know it would require assistance from the state highway department, but if we had signs with flashing lights at entry points to downtown "no through trucks, no engine breaking past this point, speed limit through downtown, these are strictly enforced " might help deter through traffic from going around the square. Signs would probably need to be before both ends of bypass, at Parkway exits, Hwy 80, etc. but it might help.

It might require multiple signs at each site, but that, plus a few citizens willing to chase and pull over offending traffic might get the word out among the trucking community that it is a hassle to go through downtown Columbia.