At the moment, the preferences survey asks for both genres one likes to read AND a comma-separated list:
"Tell us about the kind of books you like to read.
The best recommendations are generated when your answer is a simple, comma-separated list of the things you're looking for."
Presumably this enables filtering by tags not just genres.
However, when it comes to what the user DOESN'T want to read, the only option is to add genres. Perhaps a similar comma-separated list of "Tell us about the kind of books you DON'T like to read." would help?
An example of my own usage might illustrate. I have "Dystopian" selected as a genre I don't want to read, but many of the books recommended for me are described in user reviews as "dystopian". They do not have "Dystopian" as a genre, so are not excluded, but if there were a comma-separated list of tags similar to the one for likes, they might still be screened out?
Yes, we're going to offer this option and likely tie it into when we allow people to specify the types of things they don't want to read in relation to content warnings.