Certified GKH Gardening Expert
There are two methods that I would recommend. Both will completely sterilize the garden soil. If there is anything that you intend to keep in the soil, then you may have to keep pulling them up as they appear.
The first method will be boiling water. This simple trick will kill anything in the soil and give you a clean slate to work with, very quickly. You may have to apply a few times over the course of a week, but it will do the trick, fast.
The next method will be solarizing the soil. This article will explain in detail: