100 years ago we have used the .htaccess to sell memberships for restricted areas.
While i want to use kirby for static pages only i see two ways:
Use .htaccess and a cronjob to delete memberships
Create .php instead .html and add a header with membership checking for all paid pages.
Are there any examples, ideas, solutions for that?
Using .htaccess to sell memberships is an interesting idea! Never seen or heard of it before. 
But back to your question. Doing memberships can be quite tricky (regarding security and payment processes). What confuses me is the conjunction to use Kirby for static pages only and memberships at the same time. Of course, this might be possible but make things also slightly more complex IMHO.
A very basic approach would be to restrict access to certain pages. See the following cookbook for first introduction:
Kirby user accounts (without panel access) can be created manually after successful payment for instance.
A more advanced and very technical approach would be to code it yourself (including the risk when something goes bad). You can have a look at the following Plug-In using stripe for payments:
And then there are a lot of 3rd party tools out there to add memberships to a (static) page. For instance:
Cheers
Thank you.
And instead of using a new user-management, i can uise the one of kirby; probably all users are saved as files … ok.