Try to think of what the jabber is trying to accomplish. Imagine you’re the guy throwing a million jabs a round, why are you doing that and what would you stop? Your opponent spamming jabs at you means he’s trying to figure out the range and has, or trying to, seize the initiative and set the fight at his pace.
You can jab back or counter. Throw a right hand over his jab or catch him with a left hook when he steps in. Give him something to think about so he doesn’t throw jabs at will without getting punished for it.
Feint. If he’s reacting to your feints, he will have less time to throw jabs.
Don’t be in front of him at jab range. Be out of range, or step in so close that he can’t jab you, or be off to the side.
Jabs are used to collect information. Sinister has a great thread on this. You can use the opponent’s jab to gauge his range as well by picking it off.
A tactic I use to assert myself in the jabbing contest is to pick one off with my rear hand and immediately pump a double jab so every jab is answered back with two of my own. Or I change elevation and slip under his jabs and drive one of my own in his body to stop his momentum.