Will they send my paper to the opposed referee?
Some journals will ask for a list of reviewers that you would disprefer. 

We all know that there are some in academia that bear grudges and this is one mechanism to make sure they don't land in your review pool. It's also a mechanism to indicate conflicts of interest (positive and negative) that might not be apparent to outsiders (a romantic relationship for example).  

It saves time for everyone as the editors then don't have to wait for COI responses and reassignments. 

So don't view the disprefer list as a wholly negative list. There are positive reasons to disprefer someone. 

But it should be short. More than four or five names and the editor will suspect that you're trying to game the system by dispreferring all possible critics. 

And to answer your question specifically: yes, an editor may send an article to someone on your disprefer list. It is - after all, your preference - and not binding in any way on the editor. They might, however, scrutinize and weigh the response of the reviewer on the disprefer list more carefully than if they hadn't been flagged.