Try this then: For every level lower of the spell that is being cast, the counter-speller takes a -1 penalty (therefore, a caster trying to dispel a 9th level spell with a 1st level spell would take a -8 penalty) So that'd make it incredibly unlikely for it to work (Nat 20 in this case isn't an automatic success, nor is a Nat 1 an auto fail).

So it'd stack up like this (assuming the characters both start with 18 int, and the lvl 17 has put all 4 points into intelligence), then the counter-speller would take an overall penalty of -4 to the roll, while the other caster would have a +15. Soooo.... basically, the level 1 caster couldn't succeed on the check unless he got 20 AND the other caster got a 1

I dunno, I just kinda like the image of an incredibly lucky young sorcerer or wizard blocking a supreme archmage once... besides... next round, Archmage would simply cast again, and bye bye little wizard...