Just say politely exactly why you don't want to be doing one:
"I don't have the time to spend on this take home test"
"I don't think take home tests are a fair way to do interviews: they require more time from a candidate than company is putting in, and I disagree with participating in encouraging such imbalance"
Etc.
If you don't care about them learning why you are pulling out, you can give them a white lie like my first example.
But politeness is not in the reason, but in the way you express that reason.
In my experience, you should only resist or refuse a portion of an interview if you're comfortable just totally walking away from a job opportunity. I've said no to ~8ish interview components and only once was I offered an alternative. The other times I was simply removed from the candidate pool.
"I don't have the time to spend on this take home test"
"I don't think take home tests are a fair way to do interviews: they require more time from a candidate than company is putting in, and I disagree with participating in encouraging such imbalance"
Etc.
If you don't care about them learning why you are pulling out, you can give them a white lie like my first example.
But politeness is not in the reason, but in the way you express that reason.