My partner and I created a pre-employment testing website. Users come to our site, pick and choose questions they like from our library and then invite their candidates to take the test.
We (owners of the website) own the copyright to questions in our library. We would now like to limit/reduce/slow down sharing of the answers to our questions. To do that, we have stated in site's terms of service that we own the copyright to the solutions created by candidates.
When a candidate leaks their solutions online (mostly unintentionally), we would like to use these grounds to get the answer removed from the Web (DMCA take down notices) where it's possible to do that.
Is there any legal reason why we are not allowed to do this?
EDIT: Our knowledge of legal systems is very limited and some of the first attempts we tried so far resulted with statements that are putting in question our right to claim ownership over user-generated content so we wanted to quickly check if we are doing something obviously wrong.
EDIT #2: I would appreciate a recommendation for an ideally online law service that could help us sort this out (Terms of service that candidates need to agree to before taking the test) so that it's legally defensible.