I've been a DM for about 3 years, and have predominantly run one-shots and short campaigns in DnD5e and PF2e. I have a player who persistently builds primary caster based characters, but then won't do anything in combat but "I stab it with my dagger." They rarely use cantrips, and basically won't cast a leveled spell unless I suggest it immediately before their turn. They seem to enjoy playing despite the fact that they're far too squishy to be a front-line melee character and don't utilize most of their class features. I've talked with them explicitly about how their play style seems to be discordant with the kind of play they want to do, and that maybe next time they should try a paladin/champion or a fighter/rougue subclass with some minor casting. They agreed at the time that sounded like a good idea, but low and behold showed up to the next one-shot with a primary caster, and over 3 hours of play and 3 combats never cast a single spell, including a cantrip.
I enjoy playing with this persons as a whole. They are engaged in the fiction, and are particularly engaged during exploration activities. They tell me they also find combat quite fun, and they are requesting I run a mega dungeon in the near future.
As a general rule, I like to let people play how they have the most fun, but issues have arisen with this play style. Namely, all of my TPKs have been associated with this player charging a squishy character directly up to a significantly stronger villain and continuing to stab it with a dagger until they went down, significantly hindering the party in the action economy and resulting in a TPK. I feel I have to intentionally weaken all of my encounters to keep the party feasible in the face of such mechanically poor combat choices.
What else can I do to help drive this individual towards melee builds, and/or help encourage them to change their play style to better suite the caster classes they choose?
Why would a player drawn to spellcasters prefer weapon attacks over cantrips? I think that's the question we need to focus on.
Are they having trouble finding spells that fit the theme of their character? If so, maybe they'd benefit from looking at a wider range of source books. You can also remind them that they can "reflavor" their spells to better fit their character.
Is it utility magic that draws them to spellcasters? Are they even choosing offensive spells to begin with? If not, maybe you can help them out with a magic item - something to guarantee that they have access to a spell or two that would benefit the party. You could also shift some of the danger towards non-combat encounters to give their spells more value. For what it's worth I don't think there's anything wrong with utility-focused characters - they just tend not to shine as much in one-shots. Maybe if they knew more about the adventure ahead of time, they could better cater their character to the situations they're likely to face.
Are they overwhelmed by their character sheet? If so, playing short campaigns starting from level 1 might help to ease them into their class abilities. Spell cards might also be helpful if you're playing tabletop.
They generally pick combat focused spells, minimal utility cantrips/leveled spells. But then don't use them. When they have utility cantrips, they rarely use them outside of combat (like won't cast detect magic to look for things, won't use prestidigitation for intimidation, etc).
Modeling themselves after Tolkien where magic is only used in the direst of circumstances, maybe?