hippie-expand-ignore-buffers

hippie-expand-ignore-buffers is a variable defined in `hippie-exp.el'.
Its value is

("^ \\*.*\\*$" dired-mode)

Documentation:
A list specifying which buffers not to search (if not current).
Can contain both regexps matching buffer names (as strings) and major modes
(as atoms)

You can customize this variable.