I'm just guessing, but i have 2 ideas.

1. In /.config/evolution/sources directory there are some files with "source" extension. I created a few additional offline calendars by producing new .source files. Maybe the online calendar properties are stored here too. So just delete the adequate source file.
2. Try System settings > Accounts. A list have to be there that contains your online calendars. You might delete them.

@mateo_salta
Thank you Mateo! What a great and simple solution. It solved my problem. And i hadn't noticed that reinstall all apps button before. I love this community, and this OS.