Instants - are the 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.