the prune and spray method
Late in the season - from about November on - many of the foxglove plants in a patch will be starting to set seed. Even if you pull them out immediately, they will stay alive long enough to drop thousands of seeds into your soil, and put you further behind the eight ball. Or you could spray them, but not only do they soak up a huge amount of chemical (the bigger the plant the more spray it takes), chances are they'll still seed before they die. There are ways to hand-pull successfully even at this late stage, but they are very time consuming.
This is where the prune and spray technique comes in. With a pair of secateurs, cut off and collect the flowering tops. Ignore any plant that isn't going to flower and seed soon, just do the most mature ones. As always, start on the outside of the infestation where it is thinnest and work your way in towards the worst part. Make piles of the cut tops and leave them to rot.
Now you can spray. There are no flowering heads to worry about, all you have to do is coat each plant in spray.
Naturally, you'll have very thick regrowth next season in the spots where you piled up the heads, but it is much better to have a few small trouble spots than one great big one.
Prune and spray works really well when you have a team of two or three people; one or two to prune, one to come along afterwards with the sprayer.