Lander215 is right. You do not need to stop the smtp service and rename directories, etc. Here is my batch file for the exact same goal:
del /Q /F "D:\ExchSrvr\Mailroot\vsi 1\badmail\*"
It works like a champ and my smtp server is down for 0 seconds. Can anyone beat that? The more unnecessary steps you take, the more chances you have for something to go wrong. It is not uncommon for the smtp service to hang when running the net stop command. So let's say you run your script after hours and it hangs. Now you get a phone call in the morning that email isn't working. Users are down, you lose sleep, etc. Why take that chance? It isn't necessary. Good Luck.