Hornets are very territorial - if they are the same as wasps.

You can buy fake hornets nests at hardware stores. Hang it up...and if they think there are other there, they will leave and build elsewhere.

Wikianswers: All hornets are wasps; some wasps are hornets.
http://wiki.answers.com/Q/What_is_th...p_and_a_hornet

This one includes instructions on a DIY fake nest:
http://www.iheartpaws.com/forums/showthread.php?t=8300