No click on current menu
Users should not be able to click on menu items that take the user to the current page. Doing so is pointless and makes the page refresh itself. This is generally a waste of time for the user and can also be confusing if the user is expecting a new page.
It's very easy to fix this problem whilst writing your own code. You change a link element, to a span. This will prevent the user from being able to click again on the same item as it is not a link anymore.
Menu systems that are pre made by others won't always have this functionality. Therefore we need to try to change our element so it looks like it can't be clicked. Usually a class or id is added to the element such as 'active' or 'current' to allow us to style the element appropriately. If this is the case then we should always make some kind of indication that the element is associated with the current page by either changing the colour or giving the button a border.
We should further our design by changing the cursor that appears on the menu item to a pointer. This will give the illusion to the user that the element can't be clicked even though it can be. You could also create some javascript to disable the button.
You don't necessarily have to make the link unclickable, just make it look like it shouldn't be clicked.