My opponent declares his white 3/3 first strike attacking.
I declare two green 3/3 blockers stacked with the intent of trading 2 for 1.
My opponent casts Brave the Elements choosing green.
His 3/3 First Striker will deal 3 damage to one of your 3/3s in the First Strike Combat Damage Step. That creature will die right after that (SBAs are checked before a player would receive priority, and would see your 3/3 with lethal damage marked on it, then would destroy that creature). Then in the Regular Combat Damage Step, your 3/3 would try to deal its damage to his 3/3, but the damage would be prevented by the Pro Green that his creature now enjoys. His creature doesn't deal damage again because it already dealt damage in the First Strike Combat Damage Step, and it doesn't have Doublestrike.
In-case you were asking about the 'can't be blocked by X' part of protection, it doesn't apply here since the creature has already been blocked.
Giving a creature any sort of ability that affects blocking after blockers have already been declared doesn't undo the block(s) that were made. This goes for anything from Flying to Intimidate and in this case, protection.
I declare two green 3/3 blockers stacked with the intent of trading 2 for 1.
My opponent casts Brave the Elements choosing green.
What happens next??
Modern
RBig RedR
GMean GreenG
WWW AlliesW
BGScavengeBG
WUVenser SilenceWU
EDH
RWAurelia 1 vs 1RW
GWURoonGWU
GWSaffiGW
Giving a creature any sort of ability that affects blocking after blockers have already been declared doesn't undo the block(s) that were made. This goes for anything from Flying to Intimidate and in this case, protection.