Neighbor B needs to, politely and in a friendly manner, explain to Neighbor A that his yard needs to be off limits to the canine offenders.
He needs to tell Neighbor A that he would like any leavings by said dogs to be removed at the time they are deposited, and if it is not done so then maybe a payment schedule to the tune of $10 per pile could be instituted.
I believe that people can, and should, work things out themselves without “authorities” getting involved. Only after friendly and polite attempts to resolve these kinds of disputes should it be escalated.
I’m surprised Neighbor B has not procured, and begun to use, a BB gun on the offending critters. I’m pretty sure he would modify their behavior rather expeditiously!