This is dependent on the computer, as not all computers have an external mic in socket, though there may be workarounds even for them.
Someone above pointed out that for some macbooks there is an ifixit explanation of how to replace the mic, where the suggestion was made to simply disconnect it, and don't put anything new in it's place. If you are worried about this voiding the warrantee here are a few other suggestions.
Check to see if plugging in an external mike capable headset into the audio jack/socket switches the audio in to the headset. If so, you should be able to use the mute switch on the headset to disable the mic by default. If the mic is only disabled if there is a connection from the mic ring on a 4 conductor jack to ground, and only if there is the appropriate resistance, you can make that connection through an appropriate valued resistor. This may need to be done on a cable that supports external speakers if you have a single audio plug that supports both audio out and audio in, presuming you want to hear the audio of your computer.
If you want to be able to make use of the mic on the laptop, you should be able to simply unplug that plug, and the computer (most modern computers in the past 10 years anyway) should switch back to the internal mic. Alternatively switch to a headset that has a mic from the cable that disables the mic. Or if your computer supports a bluetooth phone headset, you should be able to use that for audio.
Another alternative that may automatically disable the internal audio i/o is to plug a usb sound adapter into the computer, or a hub. (Or plug in a usb hub/doc that has audio capabilities.)
You could also plug in a white noise generator, or an mp3 player with a continuous loop of someone providing a boring recitation of the constitution. If the mic port is "available" you could tape an in-ear speaker to the port with one of those feeding it audio. The down side here is that it is possible to 'learn' what you are feeding it, and cancle it out, boost the gain on the rest to recover what people are saying near the computer. You may need to provide a randomized, or mixed feed that goes through a long period between repititions, but changes rapidly. You may need a good random number generator to drive that, but that's a different topic.
If none of those are workable options for you, I'm not sure what to suggest.