I've learned a long time ago that when you need a character to die permanently, with no chance of rescue from the party,

1. It has to happen fast (from complete safety to vanished utterly in a single round, if at all possible)

2. The body has to be completely lost (taken away in a dragon's stomach, submerged into lava, planeshifted to the elemental plane of acid, whatever)

3. Ideally, it should be very clear to the observers that the character was without a doubt dead (their body being separated into different--and large--pieces is generally the only way to do this, though if you turn them undead before moving on to step 2, it can also go a long way toward selling the event)

If you do all of these things, you have an approximately 75-85% chance of the character being permanently dead. In the other 15-25% of cases, they somehow manage to miraculously figure some nonsense out, and yes, you should absolutely reward that effort; though if nobody was counting spell slots in the events that occurred in the original meme, that's honestly a fairly decent way to go about it.