You mention two different things, admin access to the router and network usage.
To restrict admin access, such as changing passwords, change the password of the admin account. You need to log into the router to do that.
There is usually no need to change the address of the router as it will be known to users anyway (otherwise the network doesn't work!), so leave it as it is if you don't have special needs.
To restrict usage of your network, you can use port blocking. Depending on your router, this may be a tedious task. What you need to do is block all ports of applications that you do not want in your network - BitTorrent typically uses Port 6881 to Port 6999, but some clients randomize their port at startup. A step further you could block everything and only allow specific ports, such as 80 for http/websurfing, Port 21 for ftp, and so on. It is very likely that functionality for your guests will be severly limited. Furthermore, if they really want to, they can always build a secure tunnel to vpn out of your net.
If you are concerned about certain websites (Porn), you can use a URL filter that blocks certain domains. However, this does not stop torrenting.
Overall, I would recommend to block the default ports for file sharing (like mentioned above) in combination with URL filtering, if that concerns you at all. But make sure your admin password is really changed!
One more thing: Keep log files. If you see something happening you don't want to see, this makes it easier to figure out what exactly happened and how to stop is from happening again. Plus, in case you get into legal trouble, you may be able to figure out whose fault it was (from a login name or mac-address). As I come to think about it, how exactly do you manage access?