You can try Killnotes.exe, but I always use task manager. Task Manager => Processes => High light “nlnotes.exe”=> End process.
Another great option is to close all other programs (if it is not too onerous to do so) and do a logoff/logon without a full restart. This will force all processes running under your Windows profile to shut down, including hung Notes processes. This has a great advantage: If Notes hasn’t actually crashed but is hung up, Windows should interrupt it and allow you to save work. This has been especially helpful in my experience. Using this technique has very beneficially since I have started using it and works with other Windows processes that like to hang in MS Excel and MS Access. I have never had it help with MS Outlook, however. When that wants to crash it just goes away with no chance for data recovery.
CNet featured the ClickGone tool. This tiny app quickly closes frozen programs.