I ask them to stop once. Then I remove them from my friends list. And in the meantime I might read one or two of their messages, but generally ignore them -- only a very few have respected my wishes and remained as friends. Draw a line and stick to it. Consistency is key.

having just been 'ghosted' in another place, my money would be on ' Hey! Thanks for the attention but i don't think this is going to work for me' or some varient of

That is entirely a matter of your own viewpoint on proper online etiquette.
Most people just ignore.
I usually write "Thanks for writing, but I am not interested".
There is so little courtesy online, and I'd like to follow Gandhi's advice "Be the change that you wish to see in the world".

Just ask them politely, if they don't comply, simply block them. Easy peasy.