If I check the box "Block message" (prevents this message from being displayed) under Interface -> Addons -> Prat -> Extra Stuff -> Filtering -> Outgoing messages. the whole message is still displayed, only the set word is either replaced or hidden, but not the complete sentence

I've fixed the issue in the latest alpha. Available from Curseforge or Wago shortly.

By "displayed" do you mean displayed in your game client or as in displayed on the recipient's device?

both. if i enter a replacement text, it will be displayed to me and to the recipient. if I replace the text with "no input" the word is also hidden for both. like I never wrote it. I would just like to completely block a message that contains such a "filtered word" so that the message cannot be sent at all.
or was the tick box meant "block message" (prevents the message from being displayed)?
like in the picture

I suspect that the option is copied over from the Inbound pattern options, and hasn't been tested against outgoing messages.

It would be weird to have an option that specially blocks message be labelled with "prevent from being displayed" rather than worded as "prevent from being sent"