How do I turn off or adjust the clever tab completion in Ubuntu (Programmable Completion)

This is called "Programmable Completion" in bash. Use <tab> to attempt programmable completion, or use M-/ to attempt the default completion (usually files and directories). See also: http://spikeypillow.com/item/30/catid/4

The most elegant way to disable programmable completion should be to add:

shopt -u progcomp

to your .bashrc file. For more details, see the official documentation about the shopt command: http://www.gnu.org/software/bash/manual/html_node/The-Shopt-Builtin.html

To change the completion behaviour, just edit /etc/bash_completion. Scroll down to where you find the list of programs and associated extensions. It should be pretty self-explanatory from there.