Broadband provider's advert claimed "Everyone's a winner" but the offer was for new customers only.
Plusnet has been rapped for "misleading" the public with its broadband advertising.
The Advertising Standards Authority (ASA) has banned an advert for the Sheffield-based provider's half price broadband, which it said "contradicted" the terms of the offer.
Several viewers complained about the promotion, which claimed "All broadband's half off" and "Everyone's a winner" despite only being available to new customers.
A second complaint related to the £13.99 monthly line rental charge required in order to benefit from the deal. This was also upheld by the ASA.
In a statement, the ASA said: "We considered, particularly given that the ad emphasised that 'All broadband's half off' ... the text 'With £13.99 monthly line rental' was likely to be interpreted [as] the price of line rental for consumers who chose to take it.
"That was a significant condition of the promotion that was not made sufficiently clear. On balance, we therefore concluded that the ad was misleading."
A spokesperson for Plusnet told broadbandchoices.co.uk: "Our recent 'Half Off' ad has not been on air since 7 February 2013, in line with our original scheduling plans.
"We did receive a small amount of negative feedback regarding the clarity of some aspects of the ad and, as a result, changes were made whilst it was still on air.
"As is always the case with our ads, the script and visuals were put through rigorous checks with our in-house legal teams and with external governing bodies.
"However, in hindsight, we do accept that some aspects of the ad were not as transparent as they could've been.
"We have no plans to reuse this ad and all feedback received will be fed into our latest campaign in order to avoid confusion for customers going forward."
Earlier this month, Plusnet's broadband rival TalkTalk was warned against "misleading" the public after its advertising claimed a YouView TV box was "free" to customers when they may actually be charged £50 for installation.