gnus-thread-expunge-below

gnus-thread-expunge-below is a variable defined in `gnus-sum.el'.
Its value is

nil

Documentation:
All threads that have a total score less than this variable will be expunged.
See `gnus-thread-score-function' for en explanation of what a
"thread score" is.

This variable is local to the summary buffers.

You can customize this variable.