reftex-bibfile-ignore-regexps

reftex-bibfile-ignore-regexps is a variable defined in `reftex-vars.el'.
Its value is

nil

Documentation:
List of regular expressions to exclude files in \bibliography{..}.
File names matched by these regexps will not be parsed by RefTeX.
Intended for files which contain only `@string' macro definitions and the
like, which are ignored by RefTeX anyway.

You can customize this variable.