Baldi - If you have an apple in your inventory, you can be safe from him. If he steps on the math book you drop, he'll stop in place to read it.
Marii - Nothing.
Slender Baldi - Use the camera flash on him.
Beepster - If he gets angry, there's nothing you can do so keep visit him and press the blue button.
Flandre Scarlet - If she's out, there's nothing you can do so keep her awareness low by collecting notebooks, use anything that makes the disorienting sound or serve Remilia the tea.
Mostly you can use the BSODA on them.