Mallow in Marlborough is very strong. We wish to eradicate some of the more established plants. What is likely to be the most effective way to tackle this problem?
There are no herbicides that keep seeds from germinating, unless you mean by killing the weed you are keeping it from reproducing. Preemergent herbicides work by putting poison in the top of the ground that kills the new growth after the seeds germinate and are growing through the treated layer.
This article covers various aspects of mallow control that you should find helpful: https://www.gardeningknowhow.com/ornamental/flowers/common-mallow/controlling-mallow-weeds.htm