Are you allowed to not block and let the damage slip through?
You are not forced to block attacking creatures.

There are certain creature abilities that force you to block, but generally, you can leave attacking creatures unblocked, and they will deal their damage to you instead of your creatures. I suggest you read the Basic rulebook:


  Your opponent chooses which of his or her creatures will
  block. Tapped creatures can’t be declared as blockers.
  For blocking, it doesn’t matter how long the creature has
  been on the battlefi eld. Each creature can block only one
  attacker, but multiple blockers can gang up on a single
  attacking creature. If that happens, the attacking player
  orders the blockers to show which is fi rst in line for
  damage, which is second, and so on. Creatures don’t have
  to block.