You can use Restricted mode. You can't install new apps in Restricted mode. When you are not in that mode, you can choose which apps to allow in it. So install what you want, set up Restricted mode, allow only the apps you want available in there, then enter Restricted mode whenever you don't want someone to be able to install apps. This would be under Security settings, Restricted Profile.

You can try TVUsage app and use it lock Google Playstore itself with a 4 digit PIN.

TVUsage app - https://play.google.com/store/apps/details?id=in.codeseed.tvusage

Disclaimer: I am the developer of TVUsage app and App lock is a paid feature.