remove or hide the "Main" or any other group tabs?

Go to Types page in Cobalt admin panel, click on selected type and you will get to edit page of type. There you will find General Parameters tab, click on it and first block on that page is dedicated to templates, where you will find Submission form and on right of it is button with tempaltes name as a title, click on it and you will see modal window. There find in General field Grouping type and select do not group and save.