[bubbacluck]

So if someone sprays silly string on me, even on purpose, I can haul off and hit them in the face as hard as I can?

I believe that I would be the one going to jail.

[fwdude]

So if someone sprays silly string on me, even on purpose, I can haul off and hit them in the face as hard as I can?

That's assault, depending on the context and surroundings. And, given you aren't in a lefty-lib cesspool of a state, you can defend yourself against an assault. Here in Texas, you would probably be no-billed for doing so, given that you're not at a birthday party where silly string is being passed out and you are present by consent.

[publius911]

So if someone sprays silly string on me, even on purpose, I can haul off and hit them in the face as hard as I can?

Depends on the nature of the crowd.

If it's a typical militant irresponsible OWS bunch of criminal losers, then yeah.

If YOU are the target, you can assume it's harmless silly string; your choice.
An LEO on the job? Not so readily. In a crowd that experience has shown there are mostly criminals, I give totally the benefit of the doubt to the cops.