Build it into the spell. Make one of the rules of the spell "if this spell was cast in any way except by a scroll scribed by Glumdyfork the Greedy, the spell has no effect." That requirement shouldn't change the level of the spell, and really is not that different from spells that can only target certain people or require specific material components. This rule would mean that even if copied into a spellbook, Glumdyfork's spell wouldn't work for that wizard.
There is nothing preventing other spellcasters from creating a similar spell on their own, but that would require them to research it on their own, which is expensive in both gold and time. You could even hide this requirement, so that unless the caster succeeds on a high arcana check, they do not realize this requirement, and the copies they make always fizzle.
Bonus points if instead you make the rule something like "Unless this spell was cast from a scroll scribed by Glumdyfork the Greedy, it instead explodes as a fireball cast at xth level centered on the caster."
If you want a RAW solution, have the wizard use a carefully worded Wish spell to cause all versions of the spell "Glumdyfork's Greedy Spell" that aren't cast by Glumdyfork or from a scroll or other device created by Glumdyfork to fail.