Hi @Robert-Stuttaford, thanks for reaching out!
You can bulk unfollow tasks using the Advanced Search feature, @lpb has shared a great workaround here:
When you multi-select your tasks, you need to select “Add collaborators” and then you will have the option to remove a specific user as collaborator of all tasks selected:
Unfortunately this doesn’t work for Conversations but I think this is possible with Flowsana, @Phil_Seeman can you confirm?
Lastly, I suggest you to add your vote to this thread Global follow-unfollow option and notifications if you wish to have this feature in Asana!
I hope this helps!