When you "erase" a disc you just do it to files - and sometimes the only thing that is done it tis to delete the file from the TOC, or the TOC (table of contents) if you do a "full erase".
However there are applications that write over the files (zeros) and replace the data with this "empty" information.
Where the format information lies is a dif story and can be dif to take it out.
There are ways to unformat a disc, but it comes with data recovery applications - intended to restore from a format and get back the previous data.
It may work if you only formated the disc once - not that sure.
If you have to buy a package to do this and considering the present cost of discs (in principle you have to use slower discs with home video recorders), is it worth the money and the trouble?