Cabergoline is a dopamine receptor agonist, meaning it stimulates these receptors the same way dopamine does. Lactotropic cells produce prolactin. These cells accept dopamine as a signal to produce less prolactin. Therefore, cabergoline creates negative feedback just like dopamine, reducing prolactin output.
From this my guess is that cabergoline reduces the output of prolactin at all times, and while spikes after ejaculation still occur, they are reduced in magnitude.