Here's a question we've gotten a lot recently: Do you ever erase palaces or actively clean them of old images?
You might consider making an active effort to erase "ghost images"--images on loci you want to reuse. The short answer is that I never actively clean palaces, whether for memory sports or learning projects. For clarity, I'll split the discussion into those two parts:
Memory Sports
In training for memory competitions, it's a necessity to reuse your stock of "competition palaces," as it would be neither practical nor wise to use a new palace for each new discipline. In his book Moonwalking with Einstein, Joshua Foer mentions that "mental athletes always halt their training a week before contests in order to do a spring cleaning of their memory palaces. They walk through them and mentally empty them of any lingering images, because in the heat of competition, the last thing you want to do is accidentally remember something you memorized last week." For instance, you might imagine each image exploding or melting into nothingness.
Although I usually rest palaces I will use in competition at least a week before the event, I don't actively do anything to clean them. I just let any old images fade with time, and I find that sufficient. In training a discipline like speed cards, I cycle through 10 palaces. By the time I've returned to palace #1 again, say a week or two later, any of the old deck's images are gone, or at most very faint. I would estimate my images only take 2-4 days to fade away. All of this means I don't have much insight into how one might actively erase palaces as part of a training strategy, but my point--for any newcomers considering this approach--is that I personally haven't found it necessary.
Learning Projects
As with memory sports, I don't actively erase old palaces whose material I've decided isn't worth remembering. I believe the alternative strategies--brainstorming and using a new palace, or using a palace so old that its images have faded organically with time--are better. (Using new palaces needn't be painful. You can find our tips here.) If creating a new, ghost-image-free palace is easy, why spend time actively erasing an old one and risk confusing yourself anyway? It's also worth noting that I try to be very selective about what I encode using memory palaces. Generally, I only encode information I'm quite sure I'll want to remember indefinitely (e.g. most high-yield medical information, language vocabulary), so having a palace I want to overwrite isn't usually an issue anyway. If, however, you're dead set on reusing an old palace, my recommendation would be to adjust each locus's vantage or focus point (e.g. if you focused on the TV's screen initially, focus on its top instead), so the loci feel different.
This is all not to definitively say it's never worthwhile to erase palaces. It's just not something I've found necessary, either in competition or learning.
If you haven't already, check out the Tips page for discussions about reusing palaces and creating palaces quickly and efficiently.