You have to incorporate into script a function that upon deleting a user it will save it's email address to prevent same use registering with same address again really need function!
Unfortunately default software does not have such functionality. In the meantime this is considered as a custom code modificationYou can do that on your own, if you have programming skills or a hire a programmer. You can do that here: http://www.oxwall.org/market/specialists Also, you can make a suggestion on uservoice: http://oxwall.uservoice.com/forums/13756-ideas-for-oxwall if it gets enough voices, we'll consider implementing it. I will move your post to the custom code modification, probably some community member will assist you with that.