I want to use my nickname as my Google+ custom URL, however Google+ is forcing me to use my name as prefix. Since I can only set my URL only once, how can I choose one without my name on it?
In short, you can't. You can only use the URLs that have been "suggested" by Google.
Your best bet to changing this policy is to send feedback.
You may be able to create a Page that is attached to your profile with your nickname as the page name. But even this is no guarantee that they will give you a vanity URL for the page.