Yes, it is possible to disable Tab completion in the MATLAB Editor and Command Window.
The functionality is documented here (see the Tips box):
You can do this as follows:
1) Click on the "Preferences" button in MATLAB's "Home" tab. This will open up the Preferences window.
2) Here, click on the 'Command Window' and 'Automatic Completions' option on the left-hand navigation pane. This will open the "MATLAB Command Window Automatic Completions Preferences" view on the right-hand side.
3) Un-check the 'Enable tab completion' sub-option:
4) Navigate to the 'Editor/Debugger' and its 'Automatic Completions' option and select the wanted behavior there for the Editor:
In earlier versions of MATLAB, this setting was found for both the Editor and the Command Window in the "Keyboard" option.