Selectively Enable Users for STream

If you remove the Stream license from a user they won't be able to login to Stream and the tile in the app launcher will go away for that user.

However as you mention below those users without a license can still Sign up for Stream to get a free trial license of Stream ad-hoc on their own. We don't have a setting in Stream to block the ad-hoc sign ups for Stream onlym but there is a way to do this if you are okay with blocking ALL ad-hoc sign ups including that of other services (Flow, PowerApps, PowerBI, etc). https://powerbi.microsoft.com/en-us/blog/ability-to-disable-free-sign-ups-for-the-free-power-bi/