First Priority: Make sure the Rules don't offer a Solution
I want to call attention to the example of the Cleric trying to knock an enemy prone, because you're framing it as the Cleric's player "trying to call a shot", but the 5th Edition rules offer a direct mechanic for knocking creatures prone: the Shove Action.
Using the Attack action, you can make a special melee attack to shove a creature, either to knock it prone or push it away from you. If you're able to make multiple attacks with the Attack action, this attack replaces one of them.
The target of your shove must be no more than one size larger than you, and it must be within your reach. You make a Strength (Athletics) check contested by the target's Strength (Athletics) or Dexterity (Acrobatics) check (the target chooses the ability to use). You succeed automatically if the target is incapacitated. If you succeed, you either knock the target prone or push it 5 feet away from you.
—Shoving a Creature, Player's Handbook, pg. 195
Just to clear up confusion, this kind of attack is substantially less good than the feature given to Battlemaster Fighters, which is better because
- It deals [a lot of] damage; a Shove deals no damage at all
- The [Strength] Saving Throw the target needs to make is harder to achieve than the ability contest made with a Shove, which allows the target to use either Strength or Dexterity, maximizing the odds they have a stat that will counter the shove.
If a player tells you they want to do something unorthodox, first check the books, since a lot of "unusual" player actions in combat have perfectly cromulent rules in place to handle them, regardless of whether the player is familiar with the rules enough to know what those rules are.
Strahd cannot be directly killed by a stake to the heart. Vampires in 5th Edition borrow many of the tropes of classical vampires, but for a Vampire in 5th Edition to be killed, it's not sufficient to stake them unless you've already successfully reduced them to 0 hit points, and stake them while they're rejuvenating in their resting place. Players are welcome to try to deal as much damage as possible with whatever tools they think will get the job done, but no amount of heart-stabbing, with or without stakes, is sufficient against a Vampire that hasn't been reduced to 0 hit points yet.
If you genuinely [think that you] need rules for how to handle "Called Shots" in general, I recommend viewing other posts on the topic, like this one, to better understand the hazards of those kinds of house-rules. But when it comes to important enemies like Strahd or other powerful boss-tier creatures/NPCs, you need to first make sure you understand the rules for how these characters can be defeated in the first place. You'll avoid these kinds of Calvinball-y situations if you have a better mastery over the scenario that's being presented to the players.