Short answer: You can't.
Long answer: You can, but you have to stop using it first. That doesn't mean just the data in /etc, it means in file ownership throughout all your filesystems. If it is used in /dev, things may get hairy. It may also mean in possible future installs, depending on your distribution. (And some distributions may restore things they stock install as you upgrade.)
As for comments: I wouldn't count on a comment syntax. (Version 7 documents no comment syntax.) You might just be changing the name. (Though I will admit, "#" comments do seem to work for me, even with busybox!)
Edit: Since you tagged "debian", I will tell you that the base-passwd package is the one that restores the stock users and groups, though it does appear to ask. It appears very little actually depends on this package, so you can probably let it get quite stale (though other packages may need the user/groups).