four ways to do it.
One, set the pop ups in pro and make every attribute in the feature read only. That is easy.
two publish two services. One as a Feature Service and one as not a feature service. Have those you do not want to edit in the not feature service. i.e. as a map image
Three from the web app configure the pop ups and make all fields not editable
Four from the web app expand the feature you do not want editable by clicking on the three dots to the right and click on disable editing.
I feel the best option is to publish two Map Services. One with edit capability and the other with out. That way you won't have to worry about settings again when you need to republish.