Are those two creatures blocked or not?
It depends on when player A casts disperse.
If player B said "I cast Give No Ground and block the two creatures", player A could say either
- "In response to the cast"
- "after it resolves but before blockers"
and cast Disperse and both creatures would be unblocked.
Otherwise, if B actually gets to declare blockers, the creatures will remain blocked.
Yes, the attacking creatures are both blocked. From the Comprehensive Rules:
509.1h An attacking creature with one or more creatures declared as blockers for it becomes a blocked creature; one with no creatures declared as blockers for it becomes an unblocked creature. This remains unchanged until the creature is removed from combat, an effect says that it becomes blocked or unblocked, or the combat phase ends, whichever comes first. A creature remains blocked even if all the creatures blocking it are removed from combat.