technically it depends on the cryptocurrency that we are talking about. if it were centralized then it is very easily possible just like it is possible to block an a PayPal account. for example government coins such as Petro or centralized altcoins like XRP are in full control of their network. at any time they can block or even take any coins they want.
Sniffing a transaction (which implies a malicious 3rd party - not the owner of the network) is generally not possible. Even not with centralized coins like XRP.
Generally, an
owner of the network can always block transactions. That's the same with bitcoin. If (and that's a really big if) all Miner would collaborate, they can block single transactions.
With BTC that's obviously way less probable than with a centralized coin like XRP, although theoretically still possible.