Delicate topic this one. First you have to want to do it. Any number of reasons :

• You hate it
• It has ruined your life, sucked up your time
• The leaders of the wiki have it out for you
• You feel ignored
• Nobody sends you messages to tell you how great your editing is
• Your articles get deleted, your edits reverted
• You're just a wiki troll
• Somebody on the site peeved you massively
• You've had it with the whole wikishit
• Collaboration is just another word for obfuscation and free labor
• Everyone's head is stuck in the clouds
• You got a job
• Your house just burned down

Yes, any number of nutter reasons might inspire you to plot the downfall of a wiki.

So, this is the why. More exciting is the how. And that's for another day. Practical lessons take time.