For years, people have tried without success to eradicate purple loosestrife. It is now clear that this is impossible and we must find ways to live with this plant. There are several, and your choice depends on the particulars of the infested site. The best control strategy is dictated by the density and extent of infestation. Be sure to get all the roots, but try not to overly disturb the surrounding soil or vegetation.
Plant cover is vital to keep loosestrife from re-invading.
All plant material should be dried and burned, or placed in a closed black plastic bag in the sun for several days where the high heat will kill the plant and seeds. Any plant material left in the field can easily re-sprout. Where permitted, plants may be treated with an approved herbicide. However, it is non-selective and will kill most vegetation it contacts. Spot sprays directed at the at the early flowering stage are most effective.