Here is a nice trick that you can use if you want to do some funs with your friends. This trick will block them from access any websites that you specify in the HOSTS file in Windows. But you only can do this trick if you can get access to his/her PC without he/she know. Let say when he/she leave the PC for awhile.
First thing that you need to do is to open HOSTS file which is located at
C:/windows/system32/drivers/etc