User Ping Problem

I have a problem with a user. Having caused problems on the network I clamped down his security on the network, but some how he has blocked me from being about to ping him. I know his IP address, but it doesnt show up on any of our logs and if I use ping it fails.
Any advice on how he did this or how i can get around it.
I need to prove more before I can go to senior bosses.