Go to the main site and then Customise (via the drop down arrow link next to 'Talk' at the top left of discussion pages) and untick the box to be notified. It should be at or towards the bottom of the list.) Then 'Save'.
Health warning. I don't know what this will do to you if you're on Mumsnet Classic - it may make you 'New' - and it does assume that you have the 'Big' Cusomise which I believe not all posters have. I don't know whether the Mobile Site has a method because I don't use that. At your own risk, therefore.