How do you restrict voting for the same category for different images?

The setting "Voting is allowed for one image only" is not available when having multiple voting categories.

Just tested this. I was able to vote for the same thing on multiple photos.

Yes, thats correct. This option is not available in combination with voting categories. These only make sense if you can vote at least in each category.

Maybe our superlative voting suits your needs. Here every voter can choose a favour image per category. You can activate superlative voting in the first step.