gnus-summary-gather-exclude-subject

gnus-summary-gather-exclude-subject is a variable defined in `gnus-sum.el'.
Its value is

"^ *$\\|^(none)$"

Documentation:
*A regexp to match subjects to be excluded from loose thread gathering.
As loose thread gathering is done on subjects only, that means that
there can be many false gatherings performed. By rooting out certain
common subjects, gathering might become saner.

You can customize this variable.