Forex.com/Gain have it in their T&Cs they do have a no debit balance guarantee - up to 50K - , however i am not sure if Oanda clearly states that .
Forex.com/Gain :
http://www.forex.com/uk/forex_understanding_margin.html
"FOREX.com: No debit balances, no margin calls
At FOREX.com, your risk is limited to funds on deposit. There are no margin calls at FOREX.com. You need to maintain sufficient funds on your account to keep your positions open, and you will not be able to open larger positions than can be supported by your account balance. If your account falls below the required level to maintain your position(s), we will automatically close out all positions to help ensure that you can't lose more money than you have in your account. FOREX.com’s Negative Balance Protection protects you up to 50,000 of the base currency of your account."
Oanda - old chat - :
Grace: Thank you for choosing OANDA. How may I help you?
marian: hi
marian: can you guarantee that a trading account with oanda will never have a negative balance, take flash crash for exmple or ny other huge gap
marian: ?
Grace: We do have a margin closeout policy which prevent the trader from losing more than you have deposited.
marian: that was not my question
marian: can you guarante i never have a negative balance when trading with oanda in any case
marian: ?
marian: yes or no
marian: would you set the account to zero? yes or no?
Grace: You won't have a negative balance by our margin policy
marian: please answer my question correctly or provide me a link
marian: its a difference to have a margin policy or uarantee that account will never have a negative balance
Grace:
http://fxtrade.oanda.com/help/policies/margin-rules
marian: my question is not answered, take this link for example, forex.com offers a guarantee up to 50k
marian:
http://www.forex.com/uk/trading-handbook-trading.html#x10
marian: we offer Negative Balance protection and will credit your account to a zero balance if your account goes into negative equity as a result of a liquidation event. This policy protects you up to 50,000 of the base currency of your account.
Grace: Just one moment please
marian: no problem
Grace:
unfortunately we don't guarantee that an account will never have a negative balance
marian: ok thank you