Logon to your router and pull up the DHCP page and look at the clients list.
Router logon:
Click Start > Run and type cmd
Type ipconfig /all and hit enter
Note the IP address listed for the Gateway
Type noted IP address into your browsers address bar and load page
Logon using your credentials saved when you set the thing up
Didn't create a password? Default logon info available in the pdf manual from the manufacturer's website.
Don't remember password?
Reset instructions for router available from above-mentioned source.
Good Luck!
PS:If the offender has file & printing sharing enabled, you may be able to connect to their PC and possible place unsavory pictures and other such nastiness on their hard drive. If they have a shared printer, you are in business for one-way communication fun!