If you put an end to a breach (in the sense of "unauthorized access", for example to a computer server), what verb could I use? Maybe, 'seal'? Or maybe simply 'close'?
You can say
We have patched the breach.
This meaning of patch is in common use in computer technology. It is a noun and also a verb, as Lexico shows:
Correct, enhance, or modify (a routine or program) by inserting a patch.
Microsoft's monthly system update schedule is called Patch Tuesday.