As a big proponent of house rules, ignoring bothersome parts of RAW and generally vivisecting rulebooks, I'm constantly approached by creative players who want to pitch in.
I'm sorry, but this won't work due to [explain the mechanical or balance inconsistencies]
It's generic, sure, but often important when the player doesn't realize an otherwise glaring problem with a significant change.
This may make DMing/planning/playing with this character more time-consuming/difficult than it's worth
Is my go-to when I'm offered a large or gimmicky change that would make playing with and around the character more trouble than it's worth. Always try offering alternatives to the ideas to "soften the blow".
Considering your edit:
Since you want to politely turn down every mechanical change, you may want to be more emphatic than the examples above, as well as let your players know your instance before the issue arises.
I understand what you want to do but, to make playing and DMing easier, I won't be accepting any house rules, homebrew content, or similar, unless they're fluff changes rather than mechanical ones. I'm taking this stance due to [explain your reasoning here, it's somewhat unclear in your question]
It's almost certain people will want an explanation as to why so be ready to talk it out with your players. It's important to let them know sooner rather than later because some players will feel disheartened that you turned them down after they spent x hours researching or thinking up these changes.