Concrete5 uses the anti-spam system in 4 blocks and tools:
Each one deals with Spam differently so it's interesting to know what to expect.
Express Form blocks will simply fail silently. The submission will be deleted, the page will reload, the form will still be filled but there will be no message explaining what just happened.
Legacy Form blocks will reload the page, delete the submission, and shows a success message.
Conversation blocks will act as if the message was posted by showing the appropriate message (success or waiting for approval). The message will be saved in the dashboard and marked as spam for the admin to manage.
Private messages will also look as if they have been posted by showing a success message. The message will not appear in the recipient's inbox.
Please keep in mind that showing a success message or not only has an effect on human submitters. Bots will have no idea what happened.