" I talked to him and said that he does nothing illegal and just access it for academic purpose. "
hAHAHAHAHA

Yea RIGHT, and I only buy naughty magazines to read the articles. IMO the best way to get them to stop is to ask them. If they are more tech knowledgeable than you, then you don't want to start a escalating of arms as you block, and they get around the block, so you block some more, etc. You shouldn't have to handicap your network just to stop one user that you are being nice enough to allow to use your network. If he gives you trouble, just change the password on the network. If he gets around that, then simply don't allow him in the house anymore.