If uploaders inform you that they have uploaded it through the generic upload wizard, this can be fixed by adding the WLM template (that automatically adds the contest categories), and in most countries also the monument identifier template. It would also be good to think about the subject categories, so that the images can be found. (But please do not add the contest categories manually, the Category:Images from Wiki Loves Monuments 2019 and Category:Images from Wiki Loves Monuments 2019 in Xyz, they only should be added by the template.)
If uploaders upload images of ineligible subjects, removing the WLM template and (in most countries) the monument identifier template is sufficient. Then a photo no longer participates in the photo contest.
(At images uploaded through the upload campaigns there is also a campaign category added, that is a category just for me to be able to track down if everything is going well, and enables me to fix something quickly if something is broken/etc. If this category is left on the file page or not does not matter usually.)
To make sure eligible images participate, and ineligible ones do not, the best moment to do this is before the jury starts their work in your country/area.