You can restrict users from logging into ZOOM to enter a meeting, and users can only enter meeting room with Meeting ID and passcode. 

1)  Go to App > App Management > Search ZOOM Cloud Meetings  

2)  Action > Edit

3)  Tap App Config, copy and paste the following scripts into the App Config box.  Tap Save.

<key>DisableLoginWithEmail</key>

<integer>1</integer>

<key>mandatory:EnableAppleLogin</key>

<integer>0</integer>

<key>DisableGoogleLogin</key>

<integer>1</integer>

<key>DisableFacebookLogin</key>

<integer>1</integer>

<key>DisableLoginWithSSO</key>

<integer>1</integer>

<key>DisableVirtualBkgnd</key>

<integer>1</integer>

        Reference：https://support.zoom.us/hc/en-us/articles/360022302612-Using-MDM-to-configure-Zoom-on-iOS

4)  Go to Device > Device Management > Bulk Action > App > VPP Install > ZOOM Cloud Meetings (You have to re-install the app even if you have already installed before)

5)  When users enter ZOOM app in the iPad, Sign In still appears but login page will be inactive as below.

Credit to : CNEC TA TUNG SCHOOL Tang Sir