When you view the resource then use Delete button you are completely removing the resource and all of the information about it. You will no longer be able to see or schedule that resource unless you enter it again. However, that does not affect your existing schedule because the details are moved to your schedule when you schedule that resource.
What you want to do is just remove it from your schedule. You can do that by view your schedule by clicking the Scheduler tab. You’ll see all of the resource you have scheduled there with a delete link near each one. That link will remove the resource from your schedule but the resource itself will still be available for later use.
If that all seems confusing, think of it this way. Scheduling a resource is like borrowing a book from your local library. When you delete it from your schedule it’s like you’ve returned the book so you’re no longer using it. But viewing a resource and deleting it is like burning the library book. It’s not there to check out again unless a new copy is acquired.
My analogy breaks down a bit because you don’t have to put the resource back to use it again, but you get the idea.
And don’t worry, you can only delete resources you’ve entered yourself.