Hi romagromov,

I am really sorry for the delay of this reply as it is a weekend for us here.

Yes, Easyblog is required category when the user submit their blog post.

Yes, if the user didn't select any category, Easyblog system will tell the user need to select one of the category only can submit the blog post.

By the way, you can set default of your category from your backend > Easyblog > category.

Sorry, but I can't find where to set default category...

Hello Romagromov,

You can click on the "star" icon to make it as default category. You can refer my screenshot here, http://screen.stackideas.com/2015-04-12_1619.png

How to remove default category?
I need authors choose category manually.

Hello Romagromov,

Hmm. Actually there is no possible settings for you to remove the default category of easyblog. However there is a workaround for this.
1) Create a dummy category called category A.
2) Set category A as default category.
3) Delete category A which is the default category.
4) Done.

Hope these help