Neem oil is natural and low toxic- but it's not a knock down spray. It messes with their system so they don't get on with their life cycle properly and eventually die out. One effect is it stops them eating, but I don't know how many days that will take or if you can bank on it- plus you'll have tons of eggs hatching all the time so will need to spray every couple days... As for something that will kill them in the short term but not be toxic? I've read a strong concoction of chili will nail them... but maybe it might burn under your nails when you pick the bud up and will have a funky taste when you smoke it- that if it doesn't just mold to high hell from spraying anything
That's why you were told to stop spraying by week 6. In some set ups with the right conditions, you can probably get away with it. Having said that, i've never met anyone who is that on top of their environment for it to be a safe consideration. Something that would help would be to put a sterylizer in the same spray like h202 and colloidal silver- but keep in mind that some chemicals don't mix well.