The ad showed Willis complaining about the speed of his current provider, with a character telling him to "try Sky Broadband, it's totally unlimited".
A voice-over then said the service was £7.50 a month, however on-screen text stated the deal was only for existing Sky TV customers.
The Advertising Standards Authority said the ad was not sufficiently clear.
Earlier in the advert, on-screen text stated consumers needed to purchase Sky Talk, calls and line rental - costing £14.50 a month - to obtain Sky's broadband service, while Sky TV prices started at £21.50 per month.
One viewer complained to the advertising watchdog, saying the commercial did not make clear the extent of the commitment customers had to make in order to receive the service advertised for £7.50.
Sky said it believed it had made its pricing clear and the average consumer would understand from the on-screen small print it was necessary to commit to its line rental and be a Sky customer.
However, it admitted it was not a requirement to be a Sky TV customer to receive the broadband package, which non-subscribers could obtain for £10 a month.
The ASA said as the advert was "clearly not directed at existing Sky customers", line rental and Sky TV would have been relevant charges to those interested in subscribing to the broadband service and it was therefore important for the information to have been stated prominently.
It said the small print at the bottom of the screen "would by its nature be significantly less prominent than a claim made in a voice-over, and was therefore not an appropriate method of communicating material information relating to the £7.50 price claim".
The watchdog added the cost associated with the TV element of the package was presented even less clearly, having disappeared from the screen 15 seconds before the voice-over.
The ASA ruled the advert must not appear in its current form again.
A TV advert for Sky broadband featuring Die Hard star Bruce Willis has been banned for being misleading.