Default Unencrypting a Jamesrom 411 Acer tablet

I shot myself in the foot and I am asking for your help.

I Encrypted my tablet, after rooting it with James rom, and after deciding that I hate encryption... I want to revert back to stock, or reinstall the custom ROM.

I found out that that's really difficult to do. Apparently my internal sd card cannot be reformatted . In recovery mode I can't access or format the SD card or the system cache. Can you help me around these problems?