If you want to remove the drop in price for customers -- and only allow those with memberships to book this class.-- you can do the following:

Navigate to the class via Classes > Schedule > edit the class > scroll to Additional Settings > under 'Restrict who is eligible to book and when' toggle on Require booking with memberships only > Save changes.

Test the class out by clicking purchase link. 'The purchase without membership' link should no longer display.