One customer complained to the Advertising Standards Authority that the ads were misleading, because the £5 refund was not made available unless further money was also staked.
In another case, a customer was refused a refund on the grounds they had not put up a big enough stake and money they "won" during a game and then subsequently lost was not counted as their own money.
Despite William Hill claiming the promotions were self-explanatory, the ASA concluded that the wording of the terms was "unclear and contradictory".
In the ruling the ASA said: "The ads must not appear again in the form complained of. We told William Hill to ensure that terms and conditions were clearly worded and unambiguous, so that consumers would understand how the promotion worked and what amounts would be refunded.
"We also told it not to describe promotions as 'risk free' if the mechanic of the relevant game meant that it was not possible in all cases to stake the required amount and receive a refund without a customer staking some additional money of their own."
William Hill said it had taken the ruling on board and said: "We will review the details, ensuring any lessons are learned with regard to future promotions."