Replaygain tags, like a few other tags, are considered calculated data that is linked to a file’s audio and does not come from MB metadata. Hence it is always merged with the data from MB when assigning a file to a recording. That way calculated replaygain stays even if the file is moved between recordings.
If you always want to remove it use a script with $delete to mark those tags for deletion. To delete all replaygain related tags known to Picard:
$delete(replaygain_album_gain)
$delete(replaygain_album_peak)
$delete(replaygain_album_range)
$delete(replaygain_track_gain)
$delete(replaygain_track_peak)
$delete(replaygain_track_range)
$delete(replaygain_reference_loudness)