Hi Dawn & welcome,
By "we" in We let our daughter's boyfriend move in" do you mean you and your spouse? If so, you might both want to be on the same page and first of all present a united front, if you no longer want your daughter and her boyfriend smoking pot in the house (or smoking anything, for that matter).
There's nothing that kids can read so quickly as how to divide and conquer the adults in their lives. Your younger kids will see how this works, too.
Does boyfriend and daughter contribute anything in the way of finances or chores to the household?
If you told them both to stop doing it and they ignored you, they clearly have no respect for you, and the younger kids will observe and perhaps imitate. You must decide what you will tolerate. I'm sorry that your generosity of spirit has landed you in this position - I know how difficult it is.