Someone else is using our Google Analytics Tracking code number. What do we do?

You can exclude the hostname sending data to your profile by setting up an exclude filter.

Alternatively, it might be better if you set up just an include filter for your own hostname to ensure you don't see anything like that in the future.

Google Analytics Filters - you might want to set up a rule/filter to "Include only traffic from the domains equal to" YourDomain.net.