It seems that a student has posted a question I have given my students as an assignment on this site.
Is there a policy regarding this?
I think the general homework policy applies. Whether the exercise-assigning person is a member of the site is irrelevant.
Any measures towards penalising use of the site would have to be taken offline at your discretion.
As a general matter, I can see four things that you might want to rectify.
If your assignment text has been posted wholesale, that may be a copyright violation. This could only apply if the wording of the assignment is sufficiently original to be covered by copyright (sometimes there is a natural way to phrase a problem, it would be difficult to claim copyright on that). If you wish, you can send a takedown notice — see “Reporting Copyright Infringements” in the terms of service. Note that copyright takedown notices have to be sent to the specified email address, site moderators are not enpowered to deal with them. If the question has answers, we would prefer if it can be rephrased rather than taken down, so a flag in addition would be appreciated.
If your assignment has been posted without attribution, and represents a non-trivial, original wording effort, it may be plagiarism. In this case, please flag the post, or edit it to add attribution. If you do not have an account on the site, you can still suggest an edit to add the attribution, or you can use the contact form to contact Stack Exchange staff who can relay a request to site moderators.
On the topic of edits, please note that an edit that adds attribution is fine (“This question was asked to the CS 142 class of Miskatonic University.”) An edit that removes content from the question or that adds editorial text such that “do not answer this, it's homework” will be treated as the vandalism it is and will be reverted.
If you are unhappy that your students do not do their homework for themselves, you should lecture them. It's not a problem that we can solve for you. We, as a community, will certainly not forbid answering a question just because it was asked as a homework question. You may mention the origin of the question in a comment (note that you need to be a user with at least 50 reputation points to leave a comment), or with an edit (see above). This will typically dissuade some, but not all, of the participants from answering fully or at all.
This particular question has been deleted by its asker. It had not had any answer.