I have managed to do this in the past, but now it does not seem to be working.
I have tried a number of different approaches using the wp_deregister_style and wp_dequeue_style within functions attached to wp_enqueue_scripts and wp_head.
I have used handles such as: buttoncss, membership-buttoncss, accountformcss, membership-accountformcss, etc. etc.
Can somebody please explain how to disable the membership css, it really doesn't suit themes giving it such bold css that in my opinion is quite dated.