Can a creature enchanted by Pacifism still block?

Pacifism enchants a creature and states that it 'can't attack or activate abilities.' Blocking is a separate game action from attacking, and Pacifism's text does not mention blocking at all. Because Magic's rules only restrict what a card explicitly restricts, a Pacified creature retains its full ability to be declared as a blocker.

Under CR 509.1, any untapped creature a player controls may be declared as a blocker during the declare blockers step unless a rule or effect says otherwise. Pacifism contains no such 'can't block' clause, so this default permission stands.

It is also worth noting that activating activated abilities (those with a colon in their cost) is separately prevented, but static and triggered abilities the creature has continue to function normally while it is enchanted by Pacifism.

Concrete example: Your opponent controls a 3/3 Bear enchanted with your Pacifism. On their turn they cannot attack with it. On your turn, if they have priority they cannot activate its abilities — but when you attack, they are completely free to declare that 3/3 as a blocker, and it will deal and receive combat damage normally.

So if your goal is to neutralize a creature entirely, Pacifism alone is insufficient; you would need an effect that also says 'can't block,' such as Arrest or Imposing Visage, to stop it from blocking.