The good news is that Google doesn’t ban domains. However, they do remove some websites from their index if they are violating Google’s Webmaster Guidelines.
If you have been de-indexed then all you need to do is correct the violation(s) and request reconsideration.
A lot of folks never bother to learn the guidelines so they haven’t a clue what is wrong when the get yanked from the index.
And then there are the folks that really like to push the limits on everything, and when they find themselves too far over the line they have no idea which of the lines they went too far over.