Silence

[Mattao19]

When do you cast it on their turn before their untap step? Or when's the best time

[Kaleo42]

No player recieves priority during the untap.

Durrin the upkeep if a trigger or other action is placed on the stack such as them casting a spell you may respond with {silence} to prevent any further actions from them.

Otherwise your next opportunity is in response to them moving to their draw step.

Lastly after they draw you may respond to them moving to their main phase.

Anything later than that is more or less pointless.

[Mattao19]

Ok cool thanks :D