Replacing from match to end-of-line

This should do what you want:

sed 's/two.*/BLAH/'

$ 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

The $ 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.