I just wrote a patch that doesn't behave quite like what you asked for, but is still much better then the current method.

What it does is this: it adds a "(unignore)" link after each user, which, when clicked on, will unignore that user.

Doing it how you asked (with a form with checkboxes) would be harder, mostly because I'd be obliged to create a new htmlcode (a callable routine) for unignoring users, and use it in the message opcode (which handles, unsurprisingly, things typed into the talk box) as well as in the form ignored users submits to. The way I've done it, I simply make that link invoke the message opcode, just as if you'd typed it in manualy (and similarly to how homenode buttons do it, it happens).

Please note that just because the patch is written does not mean it will be applied, nor applied in a timely manner. The gods do things in their own time -- which is to say, when they have the time. Additionaly, the patch may be buggy, or stylisticly bad -- I'm not the best of coders.