I don't know your reasoning for wanting them to chose to not have supernatural abilities. When playing an RPG, players almost always want to have cool powers. Otherwise, they will just be mundane while those around them do cool stuff. It sounds like you have something specific in mind for why you would want non-supernatural characters though.
Limit their choices
A really good way to ensure that some players have supernatural abilities and some don't is to limit their number of choices for class/abilities. If you have 6 players and only 3 classes to choose from, chances are more likely that each class will be used than if you provided 5 or 6 class options, even if 2 of the 3 have supernatural abilities. Along the same vein as "guiding the story" rather than "railroading", this still allows the players to make choices for themselves. Give them options, but limited options makes it clear the direction you would like the story to go without being too overbearing.
Force the classes you want
Want to ensure 4 people will have supernatural abilities and 2 won't? Create 6 character options, let them choose who to play, but only allow each class to be chosen by 1 person. This will prevent everyone from choosing the same abilities and will ensure you have the party composition you wanted.
In my experience with themed one-shots, players don't care too much about having tons of options and don't mind being railroaded a little bit to move the story along. It is more the fun of the storytelling that makes it a fun session, rather than the min-maxing options.