Can I legally force an 18-year-old child out of my house?

The short answer is "yes," but note that in Maryland a parent can be required to legally support a child through the time they graduate high school (even if they turn 18 before finishing high school).   While it is always good to get legal advice, it is unlikely that you'd need serious legal representation under these facts unless your child refuses to leave when asked. 

Yes you can.  However, if your child refuses to leave then you'll have to take legal action.  Since your child is 18 (not a minor), they will be considered to be a "licensee"; that is someone who entered and remained on the premises with permission.  Now that such permission has been revoked the lawful way to remove them from the premises is to serve them with a notice to quit (under these circumstances in some states this notice can be for a little as 3 days, in other as much as 30 days).  If they fail 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 them 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 be tempted to change the locks, etc.