The best method would be to reduce your access to your bankroll IMO and keeping the coins secure in the same time, keeping private keys in vaults in banks or secure locations would be the best IMO.
Better to cash it out, because even you are going to hide it somewhere still you can have access to it. And when you are going to crave to gamble you can just come back into it and you are going use that funds on your bankroll. But there is no always an instant way to quit gambling for good, a little by little step of quitting by doing something interesting is also a good way.
If you have no access to your private keys then you wont be able to get your bitcoin out of it. This is pretty much more convenience way just to withdraw, besides that keeping your bitcoin safe is probably one of the thing you could do to stop your addiction as well as to keep it as a source of future investment
It is just like you are burning your bitcoins. Why you would waste your bitcoins for having no access to your private keys if you can just simply cash it out and use it to some other things that you wanted to afford. Just like buying of your own new clothes or something like that. Yes keeping your bitcoin safely is a good source of future investment but when craving comes to you, you can't notice that with that purpose anymore. Because you might use that fund.