Right click a topic > Format Font > Text & Capitalisation and choose the style you require. None in your case I think.
To apply this change to all topics of the selected type in your current map, also click Map Theme at the bottom left of the pop up and select "Save in Default Theme for this map". You will need to do this for Central Topic, Main Topic and Sub Topics. If you allow different styles for more than one level of subtopic you will need to edit each level.
To make this chnage effective for all future maps, edit the default map in the Design ribbon > Template Organiser.
You can also create a Theme to apply to exisiting maps.