When you make an action, you want to achieve some desirable result. So, what are some end results this diplomat wants to achieve? To make these protests become meaningless, fade away and stop, right? So, taking a protests sign doesn't do that. Rough up a protests doesn't do that. Even though medias are already writing smear articles about China with these HK protests but now you are given them more ammos. It is exactly what these protestors want.
For me, I think the most effective way against/hurting these protestors is to stop issuing visas to these protestors (and maybe even their immediate family members) from visiting/working in HK/china for few decades.