I don't think the block goes away when it's surpassed. I think that only applies to Spell Blocks.
YS 210:
Typically, a block action lasts until the player
who initiated the block takes his next turn. At
that point, he must choose whether he wants to
take another action or if he wants to maintain
the block. There are no special rules for maintaining
blocks. Just roll the action again and take
the new result as the block strength for the next
exchange.
YS 252:
1 shift of power adds 1 to the block strength
of the block action. Three shifts of power create
a block strength of Good (+3). Any attack that
bypasses the block cancels it out.
It only specifically says this under the different examples of Spellcasting blocks
I'd like to add that a block prevents ANY skill that might be used to overcome the block. You have to determine the TYPE of action you're trying to block: movement, attacks, maneuvers.
YS: 210
When you create
a block, the block has to be specific and clear in
two ways: who it’s intended to affect, and what
types of action (attack, block, maneuver, move)
it’s trying to prevent.
So, if you're trying to use your body to Block a doorway to prevent people from getting through (block movement), the attacker might use Athletics to tumble past; use Might to Bullrush you through the door; or even Intimidate to "brush Off" and walk past...all these skills must be equal to or greater than the block. You still get to defend against those attacks normally and take the greater value...as Tsunami mentionned above.