Hide Ribbon Button

Code : 
SPRibbon ribbon = SPRibbon.GetCurrent(this);
ribbon.TrimById("Ribbon.ListItem.New.NewFolder");

Step 1 : Create web part/VisualWebpart in VS2010
add above code  

Step 2:  go to Library/List >> Site Actions >> Edit Page >> Add Web Part >> Stop Editing

see the magic..... its awesome....

Note: "Ribbon.ListItem.New.NewFolder" is the ButtonId which i want to hide from list Ribbon. 

To Find more ButtonId go to
 C:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\14\TEMPLATE\GLOBAL\xml\CMDUI.XML