Do you have specific user groups per category or just one? If you have only one user group with one category restriction then there is no need to add anything. The category field won't have any more categories to choose from but the one you have set.

Unfortunately I've got two user groups and 5 items' category.
The first user's group can add items only in one category from front-end, and the second can add items at any needed category.

So what you want is for your first group to not be able to see the other disabled categories from the category selection option?

No, it's already done.

I want to delete category selection option from front-end at all. Because from front-end users could add items only for one category.
For this I want to find file where k2 get information from form (title, alias, category, text..) and write it to the database in the k2-item table.
Thanks!

Wouldn't just hide it suffice? Why do you need to completely remove it from the core K2 files and risk a modification of such a kind?

It's not need. It's work fine with hiding now
It's just be better for my site if there wouldn't be such option on front-end adding.

Ok, I understand your position, not to edit k2 core. Maybe you are right!
Thanks, will try to find this file myself.

The file you can modify and keep in your template overrides is the itemform.php file.

If you are already in use of K2 template overrides it should be located in templates/your_template/html/com_k2/default/ folder path. If you do not use template overrides you first have to create some by copying the contents of the path components/com_k2/templates/ to your template folder in the path tempates/your_template/com_k2.