How to hide sub categories in sidebar [closed]

You didn’t give us markup so I can’t know if this is correct but

.sidebar .children { display:none;}

EDIT:
I think you’re looking to hide your .commentlist .children correct? Line 1460-1463 of your file are the .children selector. try making each of them

ol.commentlist li ul.children li.depth-2 { margin:0 0 3px; display:none}
ol.commentlist li ul.children li.depth-3 { margin:0 0 3px; display:none}
ol.commentlist li ul.children li.depth-4 { margin:0 0 3px; display:none}
ol.commentlist li ul.children li.depth-5 { margin:0 0 3px; display:none}

Is this a widget in your sidebar? I can’t tell without the markup, you only gave the .css. What is the link to your website?