It's reported one customer lost almost £100,000 after asking the company to stop sending promotions.
The regulator says the owner of Ladbrokes and Coral betting stores also didn't make sufficient checks to prevent money laundering.
The parent company, GVC, admits the compliance failures were unacceptable, and insists it has made changes.
Christian charity CARE has long campaigned on the issue of problem gambling and for more safeguarding in the industry.
CARE's Communications Manager, James Mildred said: "Gambling harm effects at least hundreds of thousands of people every day.
"Problem gambling is linked to family and relationship breakdown, job losses and in extreme cases, to suicide as well.
"But the gambling industry continues to exploit problem gamblers and it has a long history of systematic failure when it comes to acting responsibly towards excessive gambling."
Calling for further action, he said: "The failure of one of the UK's biggest gambling firms to protect vulnerable customers is a shocking reminder of how much still needs to be done to hold the gambling industry to account.
"Sadly, I don't think this is an isolated incident.
"It is absolutely right that Ladbrokes Coral is fined, but that being said, £5.9m is loose change to a major bookie and so it remains to be seen whether lessons will in fact be learned."
Earlier this year a number of Christian groups including CARE and the Church of England successfully campaigned to limit the amount people can gamble on fixed odds betting terminals.
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