This week's book giveaway is in the OCPJP forum. We're giving away four copies of OCA/OCP Java SE 7 Programmer I & II Study Guide and have Kathy Sierra & Bert Bates on-line! See this thread for details.
Well, strictly, it's just a method like any other, so yes, you can. However, the container will call destroy when the servlet or JSP is unloaded, whether you have called it already or not, so you have to be careful that you make it safe to call twice if you plan to call it yourself. What worries me though, is why you think it might be useful. Can you explain a little more what you want to do this for? It may be that we can suggest a better alternative.