Generally, unless you withdraw currencies from your account, according to terms of use, they can freeze your account and stop the payments.
Withdrawing huge sums from a hot wallet is not easy, your account will be frozen and make sure that all payments are correct, so the probability of withdrawing 1000BTC that was sent to you by mistake is zero.
Also, many platforms tighten withdrawals to ensure that your account has not been hacked.
So hypothesis for a huge amount has 0% odds.
As for the small sums, they can hire a lawyer and take it back.
[1] https://www.ft.com/content/de433d55-5cea-4929-9954-c08e5b98f004
On what grounds did Citigroup lose on?
Wrong.
Poloniex is not from USA and I think Justin Sun better known for tron, owns it now after ownership change or he is surely heavily invested in it.
I don't think they are scam, but they are not fully regulated and I don't fully trust any centralized exchange so I am not surprised with anything involved with them.
You should prove here with some screenshots how they made threats to you, but in theory they can always play the terms card if they want.
Few days ago one exchange PDAX sold customer Bitcoin for 6k by mistake and now they are asking it back just because they can, and there is nobody to stop them.
That is the risk of using exchanges.
Where is Polo based?
You mean the customer is asking for the bitcoins back?
They can also contact exchanges to which you sent tokens to and request them to block your accounts
Is around $5k a huge amount for them?
When you say authorities do you mean something illegal has been committed? I don't see how this becomes a criminal law matter unless they can prove the coins were stolen or something right?
This should be a civil legal matter if Polo wants to chase it up legally so what is this authorities tracking me down like a criminal for a mistake they admitted they done?
[moderator's note: consecutive posts merged]