There is such a thing as a "blank seed" that can remove any building or obstacle. Unfortunately, you'll have to find another hacker to do that for you.
The next time you have a stranger in your town, never let them leave without you knowing about it. When someone wants to leave, ask them to hold on, and then walk into any building. Walk back out, and look around your town for seeded objects. This "refreshes" your town and lets you see seeds that have been dropped. If nothing has happened, meet the person by the gate and watch them depart. If you find something, either prevent them from leaving by going into your inventory screen and pressing the R and L buttons repeatedly (so that the screen does not "land" on any one icon and only the background is seen), or if you don't have enough time to do that, simply turn your game off without saving. Also, if you hear the item-dropping "ploop" sound but don't see anything on the ground, that is a seed that has been dropped and will not be seen until your town is "refreshed" in the process described earlier; turn your game off immediately. Hope this helps.
Edit: If I may ask, where was the seed located? Is it in front of a dire spot, like your house or a store? I had my Able Sisters and Tom Nook shops seeded, but the hacker placed them one square too far south and I was still 'able' (ha-ha) to use them. The second time, my town gate was seeded so I had to restart my town in order to use WiFi.
« Last edited by Saint Dharumis on Jul 21st 2010 »