I had the hardest time trying to figure out how to turn off autocomplete in Aptana Studio. Which means only two things: That turning off autocomplete is very straightforward and obvious and it just took me a long time to figure it out. Or that no one has gone to the effort to explain how to do it. I’m assuming and hoping for the latter. Here’s how:

Window->Preferences->Web->HTML Files->Editor->Typing

And then uncheck everything. You should be good to go.