The only surefire way to disable your webcam is to place a little square of gaffer or electrical tape over it. Total cost: $0.01 and can be reversed in an instant.
I started doing this after seeing a guy selling widgets for it on Shark Tank. Though, I followed the shark's advice and just used a post it. But as someone whose laptop is pointed at their bed, this seemed like a necessity.
I assume you can disable the microphone with a microphone compatible headphone jack plugged in (and cut the cable off). Though it disables your speakers too.
(Doesn't help with the microphone)