I agree with Karl. Let the learners decide whether to attempt the pre-test (with the possibility of failing and then taking the course) or to just go to the full course right away.
If someone takes the pre-test and fails, that should motivate them to pay attention to the content, because it shows their lack of knowledge. Well, at least, in theory it should motivate them. There will sometimes be those, like the person described in the post, who get mad because they feel bad.
IMHO, it's good to let people fail, especially in a safe environment. Life doesn't give ribbons for "participation."