can we lock the stylesheet

For us, we have an Arbortext APTCUSTOM network directory which is locked down for access where we have all of our styles, DTDs, etc. We have around 40 writers, reviewers who can access the directory and 4 library managers who can update and control it. You might look into something like this, especially if you are using PE on a separate server since PE requires a local disk for storage of its Custom files. So our users map the network drive for their use with ATeditor which everyone has local to their machine.

As another option, you can simply right click on the stylesheet and mark it read-only.