“How do you stop psoriasis from spreading?”

Psoriasis can be treated with a variety of topical creams prescribed by your doctor. New lesions should be treated right away so they don't progress. But to keep from developing new lesions, systemic treatments may be needed.
http://davidroblesmd.blogspot.com/2014/06/psoriasis-is-chronic-inflammatory.html?view=sidebar

The most effective way to stop psoriasis from spreading is the use of prescription medications. So, I highly recommend treatment by a local dermatologist. 

Steve Schleicher