What actions do Marullus and Flavius take to correct the situation? (Caesar, Act I, Scene I)?

Answer 1

By "the situation," I suppose you mean the fact that the townspeople are out partying in the street in celebration of Caesar.

Marullus and Flavius shame them into breaking up the party by reminding them that, before Caesar was in power, they used to have the same sorts of parties in celebration of Pompey's victories. It immediately sets up one of the major themes of the play, which is that the MOB is very fickle.