Is throwing a milkshake at someone, an acceptable form of protest?
Saying what you think is acceptable. Holding up a placard is acceptable. Joining in a protest march is acceptable.
Throwing a milkshake is bullying. Legally, it's common assault. It is not NOT political discourse, it is not legally protected free speech. If you think that throwing a milkshake is an acceptable form of protest, then you also think that an appropriate reply to your milkshake, would be a milkshake thrown at you.
What happened to polite, civilised disagreement?