How can the imported entry be removed again? And is it somehow possible to do this on a site collection level only?

This post could not have been more helpful.  We were having major issues with our People Picker controls, in that they didn't at all work, and the 'Select People' dialog option in the legacy version saved the day.  Many thanks, @sean_fiene.

There exists an article in the FormsSDK how to remove the FormControl again:

https://help.nintex.com/en-US/sdks/sdk2013/FormSDK/Topics/SDK_NF_REF_PS_Remove-FormControl.htm

But this seems to be a copy&paste of the Add-Form-Article which doesn't fit to the existing parameters of the Remove-FormControl-Cmdlet. But it pushed me in the right direction to try it out.

To delete an existing FormControl, you have to do something like this in your PowerShell (at least for SharePoint 2016):

$guid = [System.Guid]::Parse("a0a89c70-0781-4bd4-8623-f73675005e12")

Remove-FormControl -Id $guid

The provided ID is the one, you can see behind the control in the Central Administration -> Nintex Administration -> Forms environment -> Form controls.

This removes the FormControl again from Central Administration and also the Legacy Controls-Section in the Forms designer.

But pay attention, it also removes the control from already existing and published Forms in the Designpane and SharePoint gives you a big error-message when calling 'New Item' in the respective list.