Well I'm a little confused, if there is no lock file then the database isn't locked in the traditional sense. If it is coming up as read only check the file properties and make sure someone didn't set the Read Only file property to true. Microsoft also has a free download called LDBview.exe which will let you view the users in the ldb file, you can get the users PC name this way. There really isn't a way to kick them out unless you have a hidden form running in the background which could do that, check the forum for examples of that.
If this is on a network drive try copying the file offline, doing a compact repair, then replacing the network copy with the compacted one.