This should do what you want:
$ echo " one two three five > four two five five six > six one two seven four" | sed 's/two.*/BLAH/' one BLAH four BLAH six one BLAH
$ is unnecessary because the
.* will finish at the end of the line anyways, and the
g at the end is unnecessary because your first match will be the first
two to the end of the line.