Poster adverts stating "it’s time to buy" Bitcoin have been banned by the Advertising Standards Authority (ASA).
They featured a cartoon image of a Bitcoin, alongside text stating: "If you’re seeing Bitcoin on the Underground, it’s time to buy."
The ASA received complaints that it failed to illustrate the risk of the investment and took advantage of consumers’ inexperience.
Luno has told the regulator that the posters would not appear again and that future ads would feature an appropriate warning about the potential risks involved.
Rules governing adverts for investments state that they must make clear their value could go down as well as go up.
The ASA said: "We understood that Bitcoin investment was complex, volatile, and could expose investors to losses and considered that stood in contrast to the impression given by the ad, that investment was simple and conventional.
"We concluded that the ad irresponsibly suggested that engaging in Bitcoin investment through Luno was straightforward and easy, particularly given that the audience it addressed, the general public, were likely to be inexperienced in their understanding of cryptocurrencies."
The ruling comes in the wake of Bitcoin – which surged in value earlier this year – experiencing a renewed bout of volatility.
It hit a record high of $65,000 (£45,891) in April but has since been hit by fears of a crackdown on the sector in China as well as Tesla boss Elon Musk’s reversal of a decision to allow the electric car maker to accept payments in the cryptocurrency.
That prompted a sharp fall last week to about $30,000 (£21,180), though on Wednesday Bitcoin climbed back to $40,000 (£28,241).
Bank of England governor Andrew Bailey has previously expressed concerns about cryptocurrencies, saying people should only invest in them if they are prepared to lose all their money.
On Monday, Mr Bailey told MPs on the Treasury select committee: "I’m sceptical about crypto-assets, frankly, because they’re dangerous and there’s a huge enthusiasm out there."