How do I prevent autofill in Apple's Numbers program?

Just keep typing, and it will overwrite the suggestion.

For example, if you have the “Bills” in your column, and you go to type the “Broncos”, the B will activate the “Bills” if you only have one “B” team. As you know, Numbers will offer “Bills” (with “ills” in blue highlight). To select that, hit enter; to ignore it, just keep typing.

Once you have several teams with similar names, when you start typing, a pop-up list should appear with your various options. Again, if you want, select one of those with your mouse or arrow keys. If you don’t want them, keep typing.

In short, that feature is basically “always on”, afaik. but you can somewhat ignore it.

Numbers—> Preferences—> General

Uncheck “Show auto-completion list in table columns”