Boyfriend won't leave; can/how to evict?

He will be considered to be a tenant if he ever contributed rent (this can even be if he paid for utilities or food).  If not, then he will be considered to be a "licensee"; that is someone who entered and remained on the premises with permission. 

In either case, now that such permission has been revoked the lawful way to remove him from the premises is to serve him with a notice to quit (depending on wheter he is a tenant or licensee), in some states this notice can be for a little as 3 days, in other as much as 30 days.  If he fails to leave at the end of that time you will have to go through the requisite procedures for eviction in your state.  At this stage the police will be most probably refuse to get involved.  Once the court enters an order for the tenant to vacate the property it will be enforced by the sheriff, using physical force if necessary.

What you need to do now is to speak with an attorney in your area.  They will advise you on the correct procedures for all of this.  If you fail to comply with these procedures you could find yourself on the receiving end of a lawsuit for unlawful eviction. So don;t use ep" such as changing the locks, etc.