Don't repeat when a button is held down

Is it possible to prevent repeating when for example ALT is down?

[Alge]

Yes, it is possible.

Is that the only function you want the Alt key to perform or are you still planning on using it as a modifier either in-game or in ISBoxer at the same time?

I'd like to use it ingame too. But if it's too hard it's not that important, I can change the button to something else ingame.

[lax]

What you can do is use the Mapped Key Wizard (from the Wizards menu) to set up a "Broadcasting Toggler", and set it to "pressed OR released" mode, and check the "This toggle should turn broadcasting OFF instead of ON" box. This will give you exactly what you're asking for, with the Hotkey of your choice. I don't recommend Alt.

Worked beautifully.