Office 2013 users are getting PoP up to upgrade to office 2016

Perhaps the purpose is to prevent the users from receiving banner to upgrade to Office Pro Plus 2016?

If yes, following steps by modifying registry could help to achieve this.

Kindly help to run Regedit from windows search box to open Registry Editor.

Please help to go to Computer\HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Office\15.0\ClickToRun\Updates

Please locate registry key UpgradeAdvertised and change its value to 0

If perhaps this registry key cannot be found, please help to create a new one with UpgradeAdvertised as its name and enter 0 as its value.

User computer may need to be restarted to make this configuration effective.

I have done some research, here are some articles I found for your reference:

keep on running Office 365 ProPlus 2013
https://support.microsoft.com/en-us/kb/3097292

Turn off the "upgrade to Office 2016" or "get the new Office" pop-up notification 

https://support.office.com/en-gb/article/Turn-off-the-upgrade-to-Office-2016-or-get-the-new-Office-pop-up-notification-1f32be31-5aea-42bb-ba3e-c753fc748d8b?ui=en-US&rs=en-GB&ad=GB