Press F9 to open your database. Click System from the navigation pane. In the top left, there is a section called "Starting Party." Ensure that section only contains the one actor that you're looking for.
To add to what others said:
Even if you deleted actor names in the Actor tab, the slots for them will still be sitting in the System tab. So you will click the "empty" slots and delete them so they don't show up in your game battles/menus.
Tomorrow is Thanksgiving, and I'm so excited! I've been exploring MZ a little, and I just downloaded Effekseer to make new animations. I have experience in 3D animation software, so I'll be pretty good at the whole "particle" thing. My first game on MZ will be Fantasy Force--starring a griffin, a dragon, and a pegasus fighting against the Dragon of Darkness.