Basically it's very simple.
You take a currency that requires 21 days for staking/unstaking, then you deposit this amount in Atomic Wallet, stake it and then you can't gamble with the crypto you're holding.
The locking (staking) is done with one single click of a button, and it's practically speaking, equivalent to activation of self-exclusion on an online gambling site with a click of a button (and yet not all sites offer it unfortunately).
For example, you stake (lock) $200 of your crypto, and that's all crypto available for you - then you can't play/gamble/risk it for at least 21 days ... do it 15 times a year time after time, and you would be excluded for a long time.
The only drawback with this method is Atomic doesn't offer this on USDT so some "gamblers" might not like this idea because they trust stablecoins more than other crypto coins, but this is FYI.
You seem to be at the polar opposite of the mentality of most gamblers, who are often impulsive and lack forward planning. I'm not sure that staking is a good solution either, because that comes with it's own unique set of risks and you have to trust the site that you are using not to disappear as well. The trouble with gamblers is they will often try to find work arounds, even if they manage to implement roadblocks like this, if they have that uncontrollable urge one day they will probably try to borrow the money - knowing that it will free up on a particular date in future. Having to self exclude 15 times in a year is also a ridiculous amount and unlikely to breed the longevity needed if someone truly wants to break this habit.