We are currently working flow and power-apps, as i can see a user with contribute access can customize the form and add a flow using SharePoint online.
how can i restrict flow and powerapps customization to only admins?
If the users has permissions to edit item, he/she will be able to create Flows attached to item.
You can assign NO Flow License in the O 365 Admin Panel. Users will get an error if they try to create a Flow.
Another workaround, check for current user and hide buttons via code: https://www.sharepointappie.nl/disabling-flow-powerapps-buttons/
Try changing the SharePoint permission level to 'contribute' and not 'edit' this will hide the 'Customize Form ' option