Trying to keep the number of table rules and exceptions to a minimum requires turning down requests for mechanical changes.
All such requests are uniformly rejected.
Thematic re-skinning of existing descriptions are fine (fluff changes), but some requests are outright mechanical changes. What is a brief canned response or block of text that can be used as a more polite alternative to, "No"?
As per NautArch:
The most polite response will not be a 'canned' response, as it will address your reasoning for saying "no". Even if the reasoning is "I am trying to limit the number of house rules" players don't like when they have an idea they think is good, but it gets shot down without justification (from their point of view).
I get the impression this response is meant for a text message or email?
As with emailing in the workplace, messages containing no punctuation, or only 'periods', are often misconstrued as having a harsher, less empathetic tone. Throw in an "I'm sorry" or an exclamation point to soften the tone of your message.
A 'canned' response that comes off much more polite would be:
I'm sorry! I appreciate your creativity, but I am limiting deviations from published texts to 're-skinning' for flavor only. This will help simplify house-keeping and maintain balance as I do not know the full implications of a change to game mechanics. The Background rules in the PHB are already set up to allow for extensive customization, so I suggest seeing if there is another Background setup which fits what you want to do.
What you say at the start of your question is a good "no" explanation that I would find hard to argue with:
At character generation, you can point them to options that they might consider instead.
Can I make a cleric that uses INT for the spell casting?
Can this character be proficient with scimitars?
This character was a spy, so should have proficiency in stealth.
Instead of coming up with some sort of universal response when approached by players' individual requests for mechanical changes (none of which you will approve) be up front with all of your players and let them know that no such changes will be allowed at any point in the game.
Then, explain your reasoning. And have everyone agree to this ruling. After that, if any requests do still come you can point everyone to the previous discussion that everyone agreed to. Thus you'll only ever have to address the issue once.
This has the added bonus of being very hard for anybody to take personally since all you are doing is addressing a general policy of yours and not negating any one person's changes.
As for what to actually say during this prima facie discussion:
As DM I would really like to keep the number of table rules and exceptions to an absolute minimum. Thus, I'm sorry, but nobody will be allowed to make any mechanical changes to the rules or their characters. I hope you all understand, but this is the way I am most comfortable running the game.
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".
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.
How about saying the following:
That's an interesting idea! My only worry is there may be unintended consequences that could throw off our campaign. I mean, D&D 5e was extensively play-tested with the rules as written - I'm not quite comfortable with tweaking the formula in my adventure I've already prepped. But I do like your suggestion. You ought to DM the next campaign and we can try your new mechanic.