Hi,
I am using K2 for 5 sites up untill now. is there a possibilty to disable "back to top" link at the bottom of an item. except for csss - display: non;?

you should find that link in item.php
Best to do an override and change the overide's item.php
Check out the videos under documentation for a tutorial

thank you. that sends me back to one of the source files. Is'nt there a possibility at the user interface to disable it?

see here - getk2.org/documentation/tutorials/item/174-templating-with-k2-and-the-concepts-of-sub-templates (there is also a video about templating)
Create your template
Look at the bottom of the item.php file for back_to_top and delete or comment it out in the override