Well Jack finally grew a pair when he pushed Flock off the pier into the water.
This was a simple but very powerful scene showing that eventhough Jack knows how incredibly dangerous and menacing Flock/MIB can be, he still had the nads to have a physical confrontation with him {albiet just a shove}.
This scene set the stage for everyone to start banning against who everybody now knows {even Clair} is a very bad person {or thing}.

Jack and Saywer are pissed and Flock is going down.