The lock feature disallows silly admins from changing the maxplayers and modifying a server password. By default, this feature is TURNED OFF so the server admin has turned it on obviously to disallow you from bypassing the server owners settings choices.
Its a good security measure for server hosts.
The only way to fix it, is to edit your Soldat.ini file and restart your server. Or ask an admin to remove '-lock 1' from their startup parameters, but I doubt they would remove it.