I'm guessing you've tried to uninstall CodeRunner by deleting the source code? This doesn't work, because the plugin is registered in the database.
The correct procedure is explained in this posting: https://coderunner.org.nz/mod/forum/discuss.php?d=535&parent=2220
The correct procedure is explained in this posting: https://coderunner.org.nz/mod/forum/discuss.php?d=535&parent=2220