There's a mana spell in Digital Grimoire which directly attacks and deactivates a focus on the physical or astral.
That said, it's perfectly viable to attack the focus and destroy it in meat space. You make some nice wooden trinket as part of your druid's kit... don't surprise if it burns in the fireball.
But from the astral... it's actually very much undefined. But I'd go with the FAQ that the best you can do from the astral is deactivate things (you used to be able to break them... which meant any kind of focus for an adept was a waste unless they also had a lot of astral combat and astral perception power). A lot of that is based on they're very high karma investments and it's a little too easy if a pack of astral spirits can suddenly surround your physical form on the astral and pummel your defenseless focus into useless scrap using astral combat.
Some of the most powerful characters in the game are people who use foci to augment their magical abiltiies... there is nothing wrong with removing a crutch if it's being abused. That said, you're far better off stealing them rather than destroying them unless it's a desperation act. Foci are worth big money even when resold for a fraction of the original price.
But if the mage brings wreck guns to a fight... he should not be surprised if his own tools are attacked and removed from the board as well. Turnabout is fair play, is one of the biggest rules I see used in groups. Do not (ab)use rules or loopholes unless you wish to be subjected to the same.
I really don't see why you bother posting in a rules sub forum at all All4BigGuns. You have nothing to comment on the rules as published. You simply comment repeatedly that the rules should be ignored and your whims honored even when the rules say otherwise. Then engage in pointless namecalling of people who disagree with you by calling faceless GM's dicks and the like.