Doing a quick search, there are a lot of questions from users that are likely victims of this sort of scam. However, given the fact that this scam preys on uninformed users, pretty much all of the question posts frame the actual problem poorly. In many cases, you have to sort of read-between-the-lines to realize OP is a scam victim.
So I think a good candidate for duplicates would describe the set up of the scam (often, the scammer sets up a blockchain wallet for the victim), as well as the 'problem' of trying to spend from a watch-only wallet.
In that light, this question is reasonable: [ I've been asked to share wallet credentials to participate in mining. Is that a scam? ] but it doesn't actually mention watch-only wallets at all (and that seems to be the point at which many users realize something is wrong, and they come here to ask a question). Perhaps it could be edited to include the watch-only-wallet part of the scam?
As another idea, this question [ What are the most notable cases of Bitcoin scams? ] could be updated to include info about scams of this nature.
@RedGrittyBrick's answer here has a list of many relevant questions:
How can I get a private key for this address