In my opinion, people can stop stanning for whatever reason they want. I just want to know how severe the hate for her currently in Vietnam? Fact as of present is that Hanni has not voiced her political stance in this matter as of right now. They can assume she can be influenced by her family however they want, but as of right now, fact is fact, they have the right to unstan, but to hate or bully Hanni online telling about them more than Hanni. Showing "patriotism/nationalism" by hating an 18 years old who had not saying anything about this matter is nut.
Also, if any Vietnamese kpop fan hating on her while stanning kpop is kind of ironic considering SK involvement in Vietnam War and if you want to dig deeper, there might be idols' grandparents involving in Vietnam War as well.