There are ways. You could, for example, set up a bbeg where that's his whole deal. The townsfolk are scared of this guy because he has the supernatural power to just kill you, straight-up. Maybe the questline leading up to their encounter involves the players finding defenses or counters or sabotaging his supply of spell components or whatever, such that, if they DO get power-word-killed, it's because they had ample opportunities to not, and failed to take them.
You absolutely do not have to RP this. You can say "No." You can say "Ok, you go off and do that, what's everyone else doing?"
What's more fun, demoting pluto or announcing the discovery of a new planet every week for a couple years?
Addl wisdom: some people make jokes when they're uncomfortable, as a defense mechanism. If you have a player or players constantly trying to lighten the mood, consider that the atmosphere you're trying to create may not be a good fit for your party, and/or parts of your party may not be a good fit for your game.
What's his deal?
Starbucks is a real coffee chain that exists in the real world. Moondeer and sunfawn follow the same naming scheme, but the players didn't realize that was what the DM was building to until the big reveal. It's...pun-adjacent.
Queercoding villains to make them seem dangerous and deviant to the people of the time (and those that are still stuck in that time). Admittedly, the people making that decision probably weren't conscious of that being why they thought eyeliner made him look villainous.
What DM would deliberately sabotage their own game like that? No one actually wants to spend a session waiting for someone to examine an endless supply of rocks.
You select the level of abstraction for different things based on what is and isn't the most fun to delve into. If your group enjoys poking every surface with a 10 foot pole, it's not wrong to play with that level of granularity. It's just that all the interesting things you can do with a 10 foot pole are pretty mined-out after 50 years so we tend to direct our attention elsewhere.
In 3.5 at least, if you're in a space that isn't big enough for you to change size, RAW you just...don't.
I just feel bad for the loincloth mimic.
Ok. It was just an example of a way you might make an encounter revolve around a spell, not an exhaustively researched adventure module.