D.F.
People have responded with good advice: marching them back with no pay-off of attention, using a gate, etc. A gate can sound mean, but you could also think of it as creating a huge play pen: their room. They've outgrown the crib (are those bars "mean"?) but they still need to be kept safe. Try to stay calm so they don't get a rise out of you. Much easier said than done!