How do I uninstall or downgrade from OS X Lion Server?
Since the server component changes files and preferences, you can't easily undo those changes other than by a back up. There is no uninstall - and there are in general no way to undo any patch or upgrade - even between major revisions like 10.6.7 to 10.6.8.

The only sure way is to restore those settings and files that get changed from a backup or set things up after a totally clean installation.

Boot into recovery, erase the volume and install Lion (without the server component) and you will have a correct and complete non server Lion. You can migrate data in from external drives, time machine (at any point in time) or make the set up new by hand.

Lion will not allow a simple re-install to "undo" the server changes as it's not programmed to do all that work in reverse.