How can membership plugin work even if it is deactivated?

Hi and happy Sunday to all,

I am sorry that I have to bring this subject up again, but I have some problems with your membership plugin. I deactivated the plugin since it did not allow visitor to go to checkout in my webshop even if I had created an access level similar to that I used for those who wanted to register.
After deactivating it I actually had my first two sales, but the funny thing is that now my visitors can actually both register, log in AND also go thru checkout with out any of the two mentioned options.

How can I set up the plugin so this would work like that for real with the other features activated also?

I would really appreaciate your help and you a very welcome to come and have a look at the backend of my site.

Thanks.
Gunnar.