The idea is to make the person look ridiculous. It's a unique form of censorship, in that the comment is still available to read. It sometimes draws more attention to the comment due to the challenge of reading it sans vowels. The practice may effectively eject the person from the community.
Here is what disemvoweling looks like, using the phrase, "The quick brown fox jumps over the lazy dog." Wikipedia used this example too. It turns out that the spam on this blog is pretty mild.
- Th qck brwn fx jmps vr th lzy dg
Disemvoweling, Wikipedia Kotaku’s New Tool: The Straight Razor of Disemvoweling, Kotaku How To Keep Hostile Jerks From Taking Over Your Online Community, InformationWeek