Yes, if the brand is not being used with an email sequence or newsletter series you can delete it from your brand list.
To delete a brand go to Account Settings select Email Brands.
This brings up a list of brands within the account. Click the pencil icon next to the brand to make changes or delete the brand.
This brings up the screen to edit the brand. If the brand is not being used, there will be a red delete option in the left hand side of the screen.
Click delete brand to remove the brand from the list of options.