This is hard to answer because some of slow payers do pay! I know we are not credit services, so here's what I do. Give the slow payer a price increase, reasonable not ridiculous, and then see if they bite on your higher-priced work. If they take you up on the new price continue to service the account and accept your late, but above average, payment!
Or do like I did Friday as this guy haggled me (I am like CarMax, don't haggle). Say, "gotta go make some money" and drive away waving and grinning like a politician.