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: