First of all, welcome to the forums!

I don't think there is a way to stop this when your main toon is a long way away. Perhaps you should look into using the popular FTL (focusless, targetless, leaderless) setup as I believe the technique solves your problem.

There are several tutorials available depending on what key cloner you use. This thread is a good start:

http://www.dual-boxing.com/showthread.php?t=10968

Also, if you use Jamba, the new Jamba (0.5) has a module written by olipcs called "FTL Helper" that you can configure to build the /assist or /target macro automatically which can save a lot of time and frustrations from typos.

Whatever you do, good luck and enjoy your boxing.