Method #1: Physically Detach Webcam's Cable
Now, this one is quite obvious. If you're using an external webcam, nothing is simpler than physically removing its cable from the port.
While detaching the cable, make sure you do not inadvertently jolt the device which may break it in a fall. I prefer to tie the webcam's end of the cable by a string to ensure the primary device do not receive a jolt in any case. If you're not going to use your webcam for a long time, carefully fold the entire cable and pack the device in a polythene bag or in a cardboard box.
Method #2: Tape or Cover the Webcam
Another way is to obstruct the lens of the webcam through a physical object. There are several ways to do it and it entirely depends on which specific method you prefer.Taping or covering with a paper sheet - You can either use a vinyl tape or a regular electrical tape to obstruct the cam. Another handy way is to use a paper sheet held with a small duct tape strip.

The disadvantage of using this methodology is that a mark of adhesive may remain on the device surface after removing the tape. I prefer using a paper sheet supported by a very thin strip of a duct tape.
Use webcam shields - If you can spend some bucks, get rid of messy tapes and get a professional webcam covering device which can be easily used on laptops and tablets. The advantage is that you can instantly hide your webcam without damaging the laptop surface in any way.

You can either use this magnetic webcam shield made for MacBook laptops. Its light and very easy-to-use. In case, you're using a regular laptop, try this excellent webcam cover which fits almost every standard model. These webcam covers are very easy to fix or mount and can be detached anytime without any hassle.
Method #3: Block Access Through OS Device Settings
If you're not a fan of shields and covers, a better alternative is to disable the functioning of the webcam at the operating system level. This cripples the webcam though some of the sophisticated spying apps may still be able to enable it stealthily. Still, it's one of the quickest and reliable ways to disable a webcam.Here's how to do it on a Windows computer.

Go to Control Panel → Device Manager → Imaging devices and select the webcam entry from the drop down tree node. Right-click it and select the Disable option from the context menu.
This setting instantly disables the webcam and your regular social applications cannot access it. In case, you want to enable it again, go to the same option again and select the Enable option from the context menu.
Method #4: Disable From Within BIOS Settings
Another geeky method is to disable the webcam right from within the computer's BIOS setting. More or less, almost every vendor ships similar kind of BIOS interface and settings with slight changes in names and options. In case, you're technically challenged and are not comfortable using this option, use the other methods listed here.
Depending on the vendor, entry point to the BIOS settings during the boot time can vary. Most common hot-keys for the same are the Del and F2 keys. Though you may cross check your BIOS docs on the internet. Once inside the BIOS interface, look for peripheral devices settings.
Here you can find the entry for your webcam. It can be labeled as a CMOS camera, integrated camera or simply as a webcam. Choose the Disable option for this setting. Once webcam is disabled from the BIOS, no application software can use it and it is completely safe from spying malware.
Here I must warn you about an important thing specifically applicable for devices having an integrated webcam like a laptop. Once you disable an integrated webcam through BIOS, associated microphone also gets disabled. So, don't panic if your microphone stops working after this action.
Method #5: Use an App to Lock a Webcam
And last but not the least is the easiest solution for the lazy. You can install a specialized application that proactively monitors access to the system's webcam and alerts you about the same in real time. Such apps also enable you to allow or deny access to an application which is trying to access the webcam.
One such application is Stop Being Watched which is available both for Mac and Windows systems. Once activated, it quietly runs in the background guarding your webcam from unauthorized access. It also maintains a detailed log of webcam access which can be viewed, at will.
Using this application ensure you can control each and every application which tries to access the webcam. I liked its log feature which tabulates every attempt to access the webcam.