Do you have any of: Hypermilky, Mega Milk, Milk Fountain, Vanae Milk, Honeypot? If not, it will decrease on it's own over time as long as you have over 100% fullness and eventually stop. Uthra+ can also decrease production.
Agree, this seems like a niche that is unfilled right now. I hate lactation so it's extremely annoying whenever my characters start to lactate because then I have to reload to avoid whatever it was that did it.