Paste as Plain Text
To automatically remove formatting from all items check the "Always paste as Plain Text" option in Preferences.
If you want to paste a particular item as Plain Text, select them and press ⌘ Shift↩︎ Return or right-click on them and select "Paste as Plain Text" in a context menu. You can change the "Plain Text" modifier shortcut in Preferences -> Shortcuts.
Please note that Paste does not modify your clipboard automatically and you have to paste items from it in order to remove formatting from your current clipboard content.