If you run the following command you will see that the default BookInPolicy is set to true, which means that everyone can use this resource.

 

Get-CalendarProcessing -Identity room-alias | fl *policy*

 

If you want to restrict access to this resource, set this to false.

 

Set-CalendarProcessing -Identity room-alias -AllBookInPolicy:$false

 

Then run the same command but exchange :$false with the mailbox user or the distribution group which should have access to the resource.

 

Set-CalendarProcessing -Identity room-alias -AllBookInPolicy <mailbox user/distribution group>

 

If you want to revert the changes made, run the following commands.

 

Set-CalendarProcessing -Identity room-alias -AllBookInPolicy:$true

Set CalendarProcessing -Identity room-alias -BookInPolicy:$none