As you are aware that farm administrators can determine whether users can purchase apps from the SharePoint Store. This setting is at the web application scope. If users cannot purchase apps, they can still browse the SharePoint Store, and request an app. Farm administrators and the App Catalog site owner can view and respond to app requests.
In our company, we are still developing SharePoint App governance place and we have disabled the end users to get apps from the SharePoint store. Since there is limited procedure to approve the app purchase, we will share the detailed approval process and some tips. Here are the whole detailed process how apps will be accessed by end users.
First, farm admin(s) can configure “Should end users be able to get apps from the SharePoint Store?” as “No” to prevent purchases but allow users to request apps as described in Microsoft technet:
Second, the site owner clicks Request It for any app, they will need to specify licensing options and an optional “request justification” field. Once the site owner submits the request, it will show up in the App Catalog’s “App Requests” List as status "Pending":
Third, App catalog Admin(s) can approve or decline app request from App catalog site by changing the status to "Approved":
Forth, App Catalog Admin will need to go to the SharePoint Store and acquire the app. This is done by clicking the link next to View App Details on the App Request entry:
Fifth, once this is done the site owner can check the “Your Requests” list and notice the status of their request. After the App has been acquired and approved the App will show up in the “Apps You Can Add” list as shown in the second step.
At this time, users could add the app to the site:
We have found you cannot add the app again if the app already added. It will show "You can't add app here".
I'm try to find a way to report whether all apps are added and way to remove them from the site. I'll keep you posted the findings.