what is the best strategy to deal with a situation where a student asks all of homework problems on an online forum
I am not sure what my opinion of this really is.

On one hand is asking on the internet really that different from asking a friend how to do a particular piece of homework, or even just looking up the method in a textbook.

Presumably you think that looking the answer up on the internet will not result in your students learning the material as well working out how to do it for themselves. The result of this is that the students will do worse in their final exams which are worth distinctly more than 10%.

You could give your students a gentle reminder of this at the start of the next lecture/course. Hopefully, they will listen to you and try and do the work on their own. If not, I wouldn't feel bad for them if they did poorly in their exams. 

Some people may be concerned that not punishing this behavior is encouraging students to cheat and gain extra marks for their homework. I would say you are quite naive if you think students wouldn't ask each other how to do the homework anyway. It is very hard to police these sorts of things which is probably one of the reasons they have a relatively low weighting.

Personally, I would give homeworks zero weighting as this removes any incentive for students to try and improve their mark this way. The questions would probably still be asked but that is more because students don't like going to office hours.

While you probably could try punish the offending student under your institution's academic integrity rules, I think you will have difficulties identifying the culprit without significant effort and possibly underhand tactics from your part. Even if you could identify them I wouldn't for the reasons outlined above.