I’m not sure how the play store works but if it’s just for testing, i.e not for reviews, then you could create a version of the app that stops working when you release the final product.

However you may just want to let those users have a copy - even if you release a beta version you may want to give them a full version for their time. I wouldn’t look at it as losing customers, but more as gaining loyal fans who may be more likely to talk positively about your app (assuming it’s good) - and that is probably going to worth a lot more than you may realise.

When I released an app on the Mac app store I gave away a few copies and some of the users left a review (I didn’t ask them to) but they were very glowing reviews, and I am certain at least 50% of the reason why people were buying my app (it was just 99p, but still sold quite a few).