It's hard simon...there is a software to blog him IP to your blog, but there is no software/perfect tips to prevent him action in making comment with your name on your blog.
This is all you can do, i think :
1) Make contact with your "friend", asking for a reason, and solve it personally. Make sure don't look angry because it will make him more doing something bad.
2) Make an post on your blog and write a statement that there is a people who use your identity. Hopefully with this, people who get their blog spammed with the comment from "fake simon" will not blame you