Generally, the best time to do this is at the last possible moment, to make responses as difficult as possible. The actual "best" time may vary a bit with different circumstances however.
For preventing attacks, tapping down creatures at the end of the first main, when you receive priority, is the probably the best time. You wait as long as possible to see if your opponent wouldn't rather sacrifice some of those creatures or something, thus reducing the chance you waste your efforts tapping something he wasn't going to attack with. Any later than this, and you miss your chance.
With only Fatestitcher, you'll have no useful time to react to prevent Prodigal Sorcerer from being activated, however. The earliest you can try to lock him down is the upkeep (your first chance to tap him since he was untapped) but your opponent can respond by activating him. This does potentially force your opponent to "expend" him earlier than he'd care to- by forcing the option now, he can't be used to attack later, and maybe is forced to make other suboptimal choices.
February 29, 2016 2:59 p.m.
A quick note to add to that, there is another phase/step in which to act to prevent attackers - the Beginning of Combat step, which is right in between Main phase 1 and Declare attackers. That would be the latest possible to tap down a possible attacker.
February 29, 2016 5:57 p.m.
@nobu is it possible if he activated prodigal first, then you just respond to it?
February 29, 2016 7:16 p.m.
Since tapping is part of Prodigal's cost, it will be tapped before you can respond after your opponent announces he is using it
February 29, 2016 8:50 p.m.
Yes, what Denial048 said. Once his ability is on the stack, he's already tapped (cost are paid as part of the casting or activating process), and what you do to him won't stop it from resolving - you could even Murder him and the ability will still resolve. You'd need to counter it with something like Stifle to stop it.
February 29, 2016 9:22 p.m.
what if i respond with Merieke Ri Berit to prodigal?
March 1, 2016 9:28 a.m.
Once Prodigal Sorcerer's ability is on the stack, it doesn't matter what happens to him. You can Murder him, Ray of Command him, Swords to Plowshares him, Soul Sculptor him- attempts to directly act on him won't make a difference as far as his ability is concerned. The ability exists on the stack independent of him now. To stop it at this stage, you need to act against it directly instead of against him, with tricks like Stifle or Reroute.
If you are really desperate, there are a few tricks that can prevent this, but they aren't ones you asked about. I mention these more for the sake of discussion. Sudden Spoiling, if cast before he is activated, can prevent him (temporarily) from activating his ability because it has Split Second, which disallows him activating in response. Time Stop can exile the ability from the stack after it is activated by forcing the turn to end before it has a chance to reach resolution.
Merieke Ri Berit doesn't do anything unique in this regard- it doesn't matter if control of him changes or if he is destroyed. The original controller still controls the ability on the stack.
March 1, 2016 9:41 a.m.
but he's is still killed right? due to merieke controlling him before merieke dies?