Grant Add Sermon but not Add Serie permission

Dear Thomas,
I am creating a system which lets some staff add sermons only but not serie and speakers, etc.
I created a form renderField('series_id') which gives a series list for selection which is good. But the user could also create a new serie by clicking the "Plus" button next to the list. It is something I want to prohibit.
I know the permission system is not that granular to set permission on sermons and series. Then I try to remove the "Plus" button so that they cannot do that easily.
How can I achieve this?
Thank you.

The simplest thing would be to hide the button using CSS. Eg
You could also create a template override for the edit view, but since it's a formfield you want to alter, you need quite a bit knowledge how forms work in Joomla.