I am having 2 problems I cannot find a solution to and mabye you could help me. In my website I have some products in K2 and when a user is not logged in the sorting doesn't work . When the user has logged in, sorting works perfectly.
And also another question. I've seen how you can add extra fields in sorting but how can you hide pre-existing fields like best or random? I really need to do this.
Thank you in advance for your help.

Where do you apply sorting to your items? I guess you refer the original sorting feature within you category settings or menu item.
Do you use any specific extension??

Please provide more details to know more about your issue.

I've seen how you can add extra fields in sorting but how can you hide pre-existing fields like best or random?

This is not default K2 functionality. By default you cannot add new sorting options.

when a user is not logged in the sorting doesn't work

This is possibly due to a cache setting. Make sure the system cache plugin always renders last.

Thank you for your replies.

1)I use the K2 extra order module. (getk2.org/extend/extensions/602-k2-extra-order-sorting).
The problem is at the front end. I have some K2 articles and I have created a 'K2 categories' menu item in order to be able to see them all. When i go to that page in the front end there are some sorting options like date, title, random etc. What I want to know is how to remove some of them. I don't want, for example, the user to be able to sort my articles randomly. How do I remove some of these options?

2)Also, I have only found 1 component named cache manager and I cannot seem to be able to find any settings to make it render last (if that is the problem). Any other ideas?

You should address these issues to the extension developer since we are not affiliated with that product.