Is there any idiom that means "do as you wish, but I warned you, so don't complain about consequences"?
Fairly informal, and rather cutting is
That's/It's your funeral!
something that you say that means that if someone suffers bad results from their actions, it will be that person's fault, not yours
On your own head be it!
Is less informal and can be less awkward.
What is the meaning of the phrase 'on your [own] head be it'?
It means that you must take the responsibility. It is usually used when someone is about to say or do something which another thinks is unwise. He will advise against, but typically say 'but on your own head be it'.
Another interesting addition, though with some nuance, is "I wash my hands of it".
If you wash your hands of something that you were previously responsible for, you intentionally stop being involved in it or connected with it in any way Cambridge
So this can be used if you were working on something with someone, and want to make it clear that you no longer want anything to do with it.
"Do as you wish, but I wash my hands of it".
This is a fairly specific use-case. In general I would go with "On your head be it".
A shorter expression than other answers, but with pretty much the exact meaning desired is suit yourself
"You do you" carries a meaning similar to that.
This short statement can be subtle or overt, depending on the tone it is spoken in. It implies a negative judgement of the other person's proposed course of action, but not to the degree that the speaker is prepared to expend effort stopping them from undertaking it.
I also might use a future rendition of the idiom:
which is a saying that has been around a while.
You're on / skating on thin ice.
on thin ice In a precarious or risky position, as in After failing the midterm, he was on thin ice with his math teacher. This metaphor is often rounded out as skate on thin ice, as in He knew he was skating on thin ice when he took his rent money with him to the racetrack. This idiom, which alludes to the danger that treading on thin ice will cause it to break, was first used figuratively by Ralph Waldo Emerson in his essay Prudence (1841): "In skating over thin ice our safety is in our speed." Christine Ammer; The American Heritage Dictionary of Idioms (2013)
If you say that someone is on thin ice or is skating on thin ice, you mean that they are doing something risky that may have serious or unpleasant consequences. Collins
be skating on thin ice
Engaged in some activity or behavior that is very risky, dangerous, or likely to cause a lot of trouble. In a precarious or risky situation. Farlex Dictionary of Idioms
This word has a lot of uses/definitions, but it can be used after giving up an argument and letting someone do what they want: whatever.
Whatever is a slang term meaning "whatever you say" , "I don't care what you say" or "what will be will be". The term is used either to dismiss a previous statement and express indifference or in affirmation of a previous statement as "whatever will be will be". An interjection of "whatever" can be considered offensive and impolite or it can be considered affirming. In the late 20th century and early 21st century, the word became a sentence in its own right; in effect an interjection, often but not always, used as a passive-aggressive conversational blocking tool, leaving the responder without a convincing retort.
Used after trying to explain something to someone, it effective means "whatever you want to do, it's not my fault."
It's not automatically understood to be that definition in all situations, so it needs context as to what it ultimately means.