How can I hide post title from Categories menu in side bar?

I hope there is someone who can help me about this. This problem makes me crazy since two days. I added “Categories” widget to my sidebar, I have 3 level categories and the 4th one is blog posts. Under “Categories” widget in my sidebar I want to show categories and sub-categories but I don’t want to show blog posts or titles there. When I click on the bottom level sub-caregory name then I want to see the whole posts in the page. I tried to explain in a picture that I attached to this question. I tried almost all the plugins related to create “collapsible category menu in side bar” but they don’t have a option to hide blog post title. By the I use Wallpress theme.