No. See Wiki:WhyWikiWorks ...
However, there is no preventing people from inserting insidious half-truths and deceptive statements into text. (Yes there is. No there isn't. Whatever.) Modifying information to misrepresent is a much more serious issue than deletion. For instance, lies sandwiched between some plausible content may not get corrected until a discerning or better informed person sees it and points it out (this may take a while, depending on the popularity of a particular page).
The deal is that people might look at the RecentChanges page for a given wiki and thus will see if a weird statement has just been added to a text, and whether or not that statements purpose is malicious. That's hard work though, so don't be malicious, be kind. It's a lot more fun.
The question above is one of the most frequently asked, and the response is shocking to many people.
In the earliest days of multiuser and networked computing there were many people who did not believe in passwords or other security measures, usually in academic or research communities. (The MIT computer labs had an especially strong open-system tradition, with unusual solutions to the social problems. One example was a crash-system command--it took away the enjoyment of figuring out how to crash the system.) Another open system is Usenet--even today there is only limited protection against vandalism, yet Usenet is still popular.
The Internet has ironically closed many of the last open resources, since policymakers required at least basic security measures for Internet-connected computers. (In one famous example, RichardStallman was given the choice of using a password (more than 0 characters) or losing his MIT account.)
It is still an unresolved (open? :-) question whether Wiki will survive--many other open communities have closed, ceased to operate, or simply failed. (See many of the WikiClones for examples.) Several people believe that the Wiki community here is fragile, and would not withstand major exposure (like the dreaded SlashDot Effect).
In any case, the Wiki is open and alive right now, thanks to the many people who have worked to build the community.