ive never heard of foxgloves killing off other plants before .There must be something else doing it like heat.
Have you been watering the plants well?
Could you perhaps show us a picture and we could perhaps help out that way

Foxgloves are grown as a companion plant in the vegetable garden as they stimulate growth and help the veg resist disease and improves storage quality.

I have grown foxgloves for a long time now, and have never had a problem. However, they have self seeded this year to such an extent that they have crowded out my salad bed! That may be your problem..