If I steal another mage's focus can I destroy his bond to it?

Also, if I use Masking can I use another mage's foci by taking on that mage's signature?

Yes, if you pay karma.

Not to my knowledge. IIRC the only magical construct you can "spoof" is a ward.

Thanks again Dakka. Do you know of a way to simply destroy the bond without killing the mage or destroying the item?

Thereby binding the Foci to yourself.

However, his bond would only be of use for Ritual Spellcasting (if you have the metamagic for it), possibly using a Spirit to Search for the guy, and that's about it. A foci only works if it is against the skin or on the body (or something). Pockets work, backpacks work, but as soon as you put down your backpack with your spellcasting focus in it, the focus powers down.

Have someone else bond it.

See above. Any bonded focus can be rebonded by another awakened character if the ritual is performed and the Karma spent. I do not know if there is a way to sever the bond without rebonding it.

There are a couple of old metagics - Focus Blocking (Awakenings) and Severing (State of the Art: 2063) that could be used to temporarily shut down a focus and/or prevent it from being used as a link back to the target. Doesn't break the tie, just frays it a little.

And of course, not available in SR4... *sigh*

Apparently we need SOTA 2073.

Disrupt (Focus) also temporarily deactivates a focus, but the bond is still intact. What about inserting it into a region with a background count high enough to reduce its Force to zero? There should be no bond until it is removed from the background count (or vice versa).

I guess not having an easy to to permanently break a bond makes sense to me. Karma is hard to come by, having ways to just destroy someones karma expenditure would be harsh, like lose limbs harsh, and losing limbs is out of the rules without optional rules. Anyways it seems there is a way to make someone lose there karma expenditure in the rules, you just got to spend karma to do it, so bind the focus already.

Thermite works too.

Yes. Destroying the focus works as well.
But I suspect the intent was to break the bond while keeping the focus in a salable state which is why the bonding idea is not working for the OP.