You have many other options, but like trying to kill the
System process, most of them are bad ideas too. (And it's unlikely that kicking every user off is going to help either, by the way.)
First of all, it's a database file. So there's a file lock on it because something, like the database engine, is reading from or writing to that file. Incidentally, this is what happens 100% of the time that a database is online or available. The database file is locked. It should be; it needs to be. You really shouldn't be doing file-level operations on a database file unless you really know what you're doing anyway, which, pardon me for observing, doesn't seem like a description that fits you.
If you must do file level operations on a database, the proper solution is to use the database management console and/or SQL/ODBC queries to take the database offline and/or "detach" it. You can do this while the database is online only if you have clustering, mirroring or some other HA functionality turned on. (If you have to ask, you don't. Or should pray very hard that you don't.) Once the database is not in use any more, you can play around with the file to your heart's content, though again, you shouldn't unless you really know what you're doing.
Should that not be possible, which happens from time to time, the next best thing is to terminate the database engine, which will be running as a service. It should be possible to do so from the command line with
net stop or through the GUI MMC snap-in
services.msc, or as a last resort, through task manager by ending the process. This should release the lock and allow you to do whatever it is with the file you're contemplating doing, which, again, is probably a bad idea. If the service can't be stopped and the process can't be killed, you're looking at a system reboot, as the service is hung waiting on a kernel resource, and won't release the file lock.
Now, before you rush off and try to kill the process, maybe another question is in order about how to achieve what you're ultimately trying to achieve. Would seem to be the wiser approach, at least to me.