recentf-exclude

recentf-exclude is a variable defined in `recentf.el'.
Its value is

nil

Documentation:
List of regexps and predicates for filenames excluded from the recent list.
When a filename matches any of the regexps or satisfies any of the
predicates it is excluded from the recent list.
A predicate is a function that is passed a filename to check and that
must return non-nil to exclude it.

You can customize this variable.