I have noticed an unidentified user using my network and I would like to know how to block him from accessing my network. changing the password has not worked.
also how to block certain websites.

Turn the router off.
Cut his RJ45 cable.

I have noticed an "unidentified user" using my network and I would like to know how to block him from accessing my network. changing the password has not worked.
also how to block certain websites.

"unidentified user" noticed ? Where, how?
"block him" or her?
"from accessing my network" what network? Industry? Office? Home? WiFi? HotSpot?
"changing the password" of what?
"block certain websites" search on forum, there are dozen of topic about it, ask there.

You do not provide any significative hint.
Hire one consultant to go inspect your network..

Turn the router off.
Cut his RJ45 cable.

+1

Turn the router off.
Cut his RJ45 cable.

best solution