In my opinion, there is no leak (even the first post says it gets constant after a few hours - a leak would grow indefinitely).
Yes, a program can tell the operating system to "forget" its memory - and the operating system either forgets the stuff it can read back from files, or writes the data into the swap file (so that it can read it from there later again). I believe that's what the previous versions of avast! were doing quite frequently (or rather, too frequently). However, it's not a good thing to do - because it slows things down (when the data are needed again, they have to be read from disk - which is million times slower than just accessing the memory). It's the OS'es job to perform the memory management - programs shouldn't really interfere with that, it's not their business (they don't have the information the OS does - it knows the overall memory usage and when it's too high, it starts "forgetting" as I mentioned - but only when needed, not sooner).
So to answer the original question - there is no way to do that from avast! itself. There might be some tools that would allow you to do that - but it's a bad idea. It would look nice in the Task Manager, but you would be interfering with Windows memory management, making things slower. If you have the memory, you want it to be used - not to sit unused.