Price comparison website MoneySuperMarket has been fined £80,000 ($103,000) by the Information Commissioner's Office for sending more than seven million emails to people who had opted out of receiving its communications.
The firm said it apologised "unreservedly".
The emails regarded changes to the terms and conditions of the site.
However they also invited people to "reconsider" their opt-out.
Asking them to do this is against the law, said the ICO.
"Organisations can't get around the law by sending direct marketing dressed up as legitimate updates," said ICO enforcer Steve Eckersley in a statement.
"When people opt out of direct marketing, organisations must stop sending it, no questions asked, until such time as the consumer gives their consent. They don't get a chance to persuade people to change their minds."
MoneySuperMarket issued an apology.
"We take the protection of our customers' data and privacy very seriously," said a spokesman.
"We apologise unreservedly to the customers affected by this isolated incident and we have put measures in place to ensure it doesn't happen again."