Can the power of an attack be reduced below two?
There are no limits.

To succeed you need to roll the target number or less on two dice.  If the target number has been reduced to 1 (or less) then the roll is moot.

This isn't stated explicitly in the rules, but more importantly there are no limits placed on spending.  It is alluded to though, on page 5 in the "Spending money to Attack " section:


  For instance, if an attacker uses enough money
  and power to exceed the target’s resistance by 20, the
  defender would have to spend 20 MB to get the attacking
  roll back down to zero again!