Lead-acid battery Polarity Swap
After reading up on an article on this matter, it seems that the only way to fix this issue is to completely discharge the battery. (article) Now since lead-acids do not want to discharge completely (80% is the rated limit before damage is done to the battery), there is no "safe" way to get rid of the reverse polarity effect on the battery. 

One thing you could do, but this would ultimately lead to the destruction of the battery plates inside, is to use the battery in reverse. The battery plates are not meant to be charged in reverse, so continuous cycles of charging and discharging will destroy them, but you could maybe get a few cycles out of it. 

Overall, the best course of action is to buy a new battery.