It mainly depends of what you consider to be your actual target object.
If your target object is the vote, then you should probably POST the vote to the system because you are actually changing the state of your target object (its internal vote counter).
Despite this, it is much more likely that you consider the forum post to be the target object of your operation and, in this case, most likely a PUT/DELETE operation set is more consistent with the REST phylosophy because you are not changing/editing your forum post. You are just putting/deleting a token into a counter. So, I would support the suggestion of Vain Fellowman.
The fact that voting should be an idempotent operation is another clue that should lead you in this direction. Think to this as a ADD/REMOVE operation over the "vote" member variable of your "forumPost" class/object.
Regarding how to tell if your real target object is the forum post or the vote, I think you should look at your code. The "object" (maybe a JSP page + a Java Controller + some Java Entity that refers to some DB table) that receives and handles this HTTP request is representing a forum post or a vote?