I teach a course, every week I assign homework. I put several hours to choose the right homework problems which cover the course material and permit them to be prepared for the exams.

I encourage students coming to my office hours and to ask all of their questions, I may also help them to solve the homework problems. During the class I normally give some hints about the homework's difficult problems (in any). 

The homework load is (in my opinion) reasonable: 4-5 exercises, maybe 3-4 out of them are easy and maybe 1-2 is a little challenging. The homework has 10% point.

Today, to my surprise I saw a user who has asked all of my homework problems (including the easy ones) on math stackexchange. And all of them have been answered. Regarding the particularity of my homework problems and the time of posting the questions, I am sure that the user is one of my students, I am not sure which one. 

To be precise I had not prohibited the students about posting the homework questions online as I was not expecting this situation. 

My question is what is the best strategy to deal with this situation.