If you don't want a sprite to be drawn, then don't call app.Draw(sprite), the simplest way to achieve this, is by having a boolean variable and before drawing, you check against the bool.
bool spriteVisible = true;
// ...
spriteVisible = false;
// ...
App Clear();
if(spriteVisible)
App.Draw(sprite);
App.Display();
// ...
Obviously if you have to do this for every single sprite, the codebase will get messier every time. That's where OOP comes into play. With a nice entitiy class and a std::vector of entities you can implement a isVisible() function and when iterating over the std::vector to draw the entities call that function first.
If you don't know anything about OOP you should definitely get a good C++ book and read it.
Btw. from App.Draw() I conclude, that you're using SFML 1.6, but I'd strongly advise you to use SFML 2, because the 1.6 version hasn't been touched in around 2.5 years, has many bugs and lacks quite a few nice features (sf::RenderTexture, sf::VertexArray, ...)