If you would like to make your subscriber select only one of your available list, one way to do so is to offer radio buttons instead of checkboxes.
Achieving this is quite easy. Add in the following:
<input type="radio" name="_mc4wp_lists" value="ecef5157f5" />
Add as many as you want, but make sure each value is unique (as this is your list’s identity). That’s it, have fun!