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.
You raised good points and in all, I advise gamblers to tackle addiction with psychological solutions and not with a restrictive solution as gambling addiction is physiological too. You may try to deny yourself today but if you are addicted will find other means to boycott those restrictions, we are humans and there will always be many alternatives, it could be borrowing, or sorting money from friends and family, the list is endless. But if we can admit that we have a gambling problem and find a lasting solution to it, we may even have the whole money in this world and still not gamble again so long as are healed.
Also, the idea shared on this thread is not so smart, what if the gambler becomes the victim of an untrusted third-party site? That is the most painful part.