It sounds to me like you and the player have an out-of-universe disagreement, but you've been trying to resolve it by using in-universe language.

The player is saying, "My character wants to assassinate the government official" (in-universe language). What they really mean is, "I want to accomplish this objective in a dangerous and exciting way" (out-of-universe language). But you're hearing this as, "I want to do something foolish and reckless."

You're saying, "The official is surrounded by armed guards" (in-universe language). What you really mean is, "I'm running a game where dangerous antics like that aren't going to work" (out-of-universe language). But your player is hearing this as, "There's an obstacle that you're going to have to surmount in order to do that."

Since it's an out-of-universe disagreement, I think you need to resolve it by talking to the player in out-of-universe terms, like this:

Player: My character wants to assassinate the government official.
DM: Well, the official is surrounded by armed guards, so that would be awfully dangerous. I'm running a game where dangerous antics like that aren't going to work; you're just going to end up getting killed. How would you feel about coming up with a safer, more conservative plan of action?

At this point, you and the player are going to have to discuss which of four options you want to go with:

1. The player thinks of a safer and more conservative plan of action, like you want.
2. You allow the player's plan to have a reasonable chance of success, like the player wants.
3. You and the player come up with a compromise between 1 and 2.
4. The player leaves the game.

It sounds to me like you and the player have an out-of-universe disagreement, but you've been trying to resolve it by using in-universe language.

The player is saying, "My character wants to assassinate the government official" (in-universe language). What they really mean is, "I want to accomplish this objective in a dangerous and exciting way" (out-of-universe language). But you're hearing this as, "I want to do something foolish and reckless."

You're saying, "The official is surrounded by armed guards" (in-universe language). What you really mean is, "I'm running a game where dangerous antics like that aren't going to work" (out-of-universe language). But your player is hearing this as, "In order to do that, you need to surmount this one particular obstacle."

I don't have enough experience to offer you specific advice. But you should try to understand what your player really wants, and you should make sure your players understand what you really want.