I have a bleu legged hermit about the size of a quater that changes shells at least once a week, he's a swinger. Some that keep there shells even though they are obviously too small, and even others that seem to just kill for pleasure (they find there way to the sump!).
I think the best bet is to have a large assortment of sizes/shapes of shells readily available. But even then, if one is eyeing up that nice piece of property on the astrea, there is nothing you can do. Either live with the deaths, or don't have hermits.
I have also found that they are more amiable to taking an empty shell if it is on its side. I had them all turned with the opening upward at first, and they never seemed to notice them( maybe i just have near sited hermits
)
Beth