You can control the available styles for the bullet-list and number-list as outlined in the documentation:
tinymce.init({
...
plugins: "advlist",
advlist_bullet_styles: "square", // only include square bullets in list
advlist_number_styles: "lower-alpha", // only include lower alpha in list
...
});
However, when there is only one style the dropdown is still present, with the single style option. There is no need for the dropdown with one item. How can you get rid of the dropdown?