Q:

How to refresh bing map?

The bing map works fine if I refresh the page and push the pin. But if I do a ajax call and load the map, the previous pins stay. How to remove those pins?

A:

Assuming that you're using the AJAX v7 control, in order to remove previous items on the map, you can use the clear() method available in the EntityCollection class.
See: http://msdn.microsoft.com/en-us/library/gg427616.aspx
So if you're using the basic way, you can use: 
map.entities.clear();

I would recommand that you're using the iSDK to start with the API: http://www.bingmapsportal.com/isdk/ajaxv7

