todo-nondiary-marker

todo-nondiary-marker is a variable defined in `todo-mode.el'.
Its value is

("[" "]")

Documentation:
List of strings surrounding item date to block diary inclusion.
The first string is inserted before the item date and must be a
non-empty string that does not match a diary date in order to
have its intended effect. The second string is inserted after
the diary date.

You can customize this variable.