As I understand, the 20 XRP serves as the payment for the space in the ledger. You can't withdraw it. No matter what, the information/transactions that have been made with that account was already stored in the ledger and you pay for it even if the account will not be used any longer.
Visit the link, a good discussion and explanation about the issue :
Fully remove all XRP from an account.Thank you, best discussion I've read on this so far. I can see that many share my concern/frustration so I'll keep an eye on the matter and revisit if/when XRP is ever worth enough to really worry about 20 coins.