How to prevent blue oxidation / corrosion from growing back?
Industrially, stuff called "conformal coating" is applied to PCBs for a variety of reasons, but it will keep moisture out and generally massively extend the time over which oxidation residue forms.

For hobbyists, clear nail polish works, though you might want to be careful so as not to allow the acetone in the nail polish to dissolve the silkscreen patterns on the PCB.

If the oxidation is not causing functional problems, you might just let it go.