It's possible to exclude a tag from the results when browsing games using a special URL right now: https://itch.io/t/160014/can-i-use-exclusion-filters
There's no way at the moment to set that from the UI, but it's something we need to add: https://github.com/itchio/itch.io/issues/738 and https://github.com/itchio/itch.io/issues/475