You can fix this. There is a special paint you can get that is especially for this problem (my father is in the paint business). With ordinary paint, damp stains will just keep coming through.
I have used this paint myself and with just one coat it covers the stain. I will ask my dad for the name of it and get back to you.
Used a spray on stain blocker and never had a problem again after it. Cannot remember name of it but any woodies or b&Q store should have similar products
Also, excuse me for stating the obvious, but just make sure that the cause of the stain has been rectified properly before doing the above! I have used white, oil based, one-coat paint, let it dry completely, then painted over it with emulsion, and it works fine. This may save the purchase of new products, if you already have spare paint in your shed/garage.