Stop spacebar keypress from triggering autocomplete in Eclipse

Eclipse 2018-12 was already patched for this!

A patch for Bug 348857 was recently merged to the Eclipse project and was released as part of Eclipse 2018-12. You can download that version here.

You now have a new option to disable all insertion triggers apart from enter, which will prevent spacebar from causing autocompletion.

Simply go to Preferences... -> Java -> Editor -> Content Assist (or Window -> Preferences -> Java -> Editor -> Content Assist on Windows) and select Disable insertion triggers except 'Enter', as shown in the screenshot below:

There is a solution on the issue tracker for this. Copy the jar in the eclipse/dropins folder. With the next restart space is diabled as autocompletion trigger.

https://bugs.eclipse.org/bugs/show_bug.cgi?id=348857

Eclipse 12-2018 and newer: This is working natively, check Pyves answer.