To prevent this is the future, you should be able to set the individual user account to restrict reactions.
And if that doesn't work (which it should), set up a separate group (probably would be easier in case many people start doing it so you would simply add them to that group called something like No Reactions allowed with that permission set to Never)? This is my "banned" account setting.
This will not clear existing ones, but prevent any future abuse.
You would need to do the above also for profile posts and any other areas that add-ons allow reactions to, which is why I would use a group setting....