Some hosts will tell you it's "wear and tear" on the sheets and an unrecoverable expense.
Some hosts will tell you to charge them for a new, full set since the set is now ruined.
Both extremes. I sit in the middle - ask the guest for the cost of trying to launder it 3 times to remove the stain - be reasonable and use any monies collected to go toward replacing the fitted sheet. You can write off the sheets on your taxes as an expense albiet, they are supposed to be amoritized over a few years. Post pictures of how the stain won't come out even now. 3 times to wash, soap and spray cleaner comes to about $10 and I would go through the Resolution Center but not necessarily asking from the security deposit.