You can't write C# from Javascript and have it be executed.
My god, could you imagine the HAVOC I could do if I could pass C# as a string and have it automatically execute?
However, you can certainly remove an object from Server-side session by creating a PageMethod or web-service that removes an object from session by it's string key.
Dave Ward @ Encosia talks about it extensively here.[
^]
It's pretty darn easy nowadays, and gets easier as you do it (in fact, it's really a tad easier thank this if you're using MVC) and really warrants looking at for further use down the road for client-interaction.