As per the title above

We occasionally have to lock a thread if it goes onto a topic we have "banned" from our forum

As a football forum we prohibit people from posting anything political, but the problem is that we could have a long thread with lots of decent posts etc, but towards the end veers off topic and people post about something else

Our usual reaction is to lock the thread so nobody else can reply, but we are having issues where a moderator will post a message explaining why the thread has been locked, but then those people that took it off topic will respond with laughing emoticons etc

I have been asked if there is a way that once a thread is locked we can prevent any new emoticons being added

There is no way in which to do this currently, however my advice would be to create a new group who are not able to use reactions. You can then easily move people who are doing that to the group in question