I am a player in said game (a certain gloura), but dont have much stock in the end decision. But here is what I would do as the caster and how I would interrupt it as the DM. First these are my assumptions.
1) The contigency spell just knows what is going on. It might not be all knowing, but pretty close.
2) The wizard does not have to know what is going on. Condition met, spell goes off. Using the blow off limb example (and not the pile of other contingencies), if you turn the mage to stone and blow off the limb, he does not know it. The spell will still go off. If it was that easy, he could just cover his ears and eyes when another player wants to do something that would set off a contingency (have to do practice drills to, so he never knows when it will really happen).
3) You can setup a contingency that interrupts something potentionally, but if the conditions are if x happens, then it will not do it before x happens.
One last step, I would like to refer everyone to the counter spell section of the SRD.
Casting Spells :: d20srd.org
Now yes, you must be aware for counterspells and have an action readied. But the counterspell listing clearly stats that there is a point when the casting of the spell is begining, but not yet cast. This is the point at which you can interceed, for counterspelling. More importantly when my contingency could go off, because contingency is always ready.
Starting simple... I would word my contingency spell so that it would cast antimagic field when "Someone other than me begins to cast Disjunction."
This would not interrupt someone from begining to cast the spell, but it would go off before they completed the spell. Similar to counterspell.
Now these instructions are not quite specific enough. We have to get more specific. You could simply change it to include "Someone other than me begins to cast Disjunction that would place me in the area of effect." or "Someone is begining to cast disjunction that would place me in the area of effect." The second is perfect as long as you dont plan on wanting to be in the disjunction. Now we are alot closer to our goal; and for me, that would be enough.
But lets take this a step further. Say the DM's interruptation is that is not specific enough. Maybe contingency cannot figure out the target of disjunction. Then I would switch to this: "When someone or something other than me begins to cast disjunction within 150ft of me."
Disjunction has a range of 25 feet +5ft/2 levels with a radius of 40 feet. Assuming a caster level of 30, that would 100 foot range with a 40 foot radius. I padded an extra 10 feet incase the caster has an ioun stone or something to boost caster level. Tell the other players and any allies you deem necessary, or add them to your list of other than's.
I believe that would cover it for spell casting, because once again:
1) Contingency knows what is going on
2) Wizard does not need to know
3) Contingency is fast, but conditions still must be met
4) There is a point before a spell is cast that things can act and contingency will if directed
It is a whole different matter I guess if there is a trap that is activated or it is a supernatural ability instead of spell or spell-like (spell-like can be countered so I am not going to lenghten my conditions to include that, I leave it to said wizard). Not to mention Epic spells and Etrudent tricks of spell emulation.
Note, I would perfer if the DM gave the party down time to buy stuff and wizard bought (or paid extra to add it to existing equipment) the original greater counterspell ring from the Arms and equipment book with disjunction cast in it. It only stops the first one, but if they are spamming disjunction, at least they are not casting some other 9th level spell at your repeatedly. No need for interrupation there.
P.S. I contingency that my Gloura PC gains a number of bonus xp no less than 5,000 xp points and no more than 10,000 xp points if my logic stands as written. Legal stuff, I reserve the right to edit anything I wrote above or within this sentence in such a fashion that it retroactively counts as the original logic written.
Ha. Editted once already.