Mets v. Nationals- HBP and warnings

[CJN]

5th inning, Pedro has hit two batters already (Nick Johnson and Jose Guillen) Guillen is up again and the first pitch hits him, Guillen starts to charge the mound, but PU Ted Barrett holds him back. The dugouts clear, but nothing really happens.

Frank Robinson comes out and has a lengthy discussion with Barrett, Tschida, and Reed, he clearly wants a warning, but doesn't get it.

So, my question is: when do you issue a warning for throwing at hitters?
In this situation it seems like a warning may be in order, simply because its the third hit batter of the night, not to mention Guillen has been hit 5 times by Pedro in his career. Whether or not Pedro was throwing at Guillen, a warning would at least quell retaliation by the Nationals.