Is your mother still paying the premiums, or are you?
If you're paying the premiums, then just contact the company and say you are no longer willing to pay and want to cancel the policy.
If she's paying the premiums, why do you care?
If the issue is the non-smoking declaration, and you are a smoker or want to take up smoking ... I don't know what country you live in, but I'd be surprised if there's a law anywhere that gives your mother the legal authority to forbid you from smoking when you are over 18. If you are paying the premiums, then contact the insurance company and tell them that you are no longer a non-smoker. The premiums may go up. If she is paying the premiums, tell her that you are now a smoker and that she should contact the insurance company about changing this clause in the policy if she wants to be sure it remains valid. If she declines to do this, that's between her and the insurance company. As you're not a party to the contract, it really has nothing to do with you. If, as you say, your mother is not fully capable of managing her affairs, you could contact the insurance company for her.
- Update based on your edit of the question *
I don't see how you could get the policy "invalidated". Unless there is some evidence that the life insurance company made false claims, or somehow tricked your mother into buying the policy, or that the policy violates local law, there is nothing "invalid" about it. Just because you've decided you don't want the policy doesn't make it invalid.
Likewise I don't see how you could get the premiums refunded. The whole point of life insurance is that you pay a monthly premium, and if and when you die, they pay the beneficiary the face value of the policy. Life insurance is often described as a kind of gambling game: The insurance company is betting that you will live long enough to pay more in premiums than they pay out in benefits. You are betting that you will get more in benefits than you pay in premiums. If you die young, you win! If you could wait and see if you die within some time period, and if not demand your premiums back, well, that would be like saying that you want to bet on a spin of the roulette wheel, and if you win you take your winnings, and if you lose you want to get your original bet back. What casino would agree to that?
You should be able to cancel the policy at any time. I suppose there might be some specific commitment in the contract, like you pledge not to cancel within X years. If you never signed it, I don't see how you could be obligated to pay for it, regardless of what commitments your mother made. I don't know where you live or your country's laws, but I doubt your mother can sign a contract legally committing you to pay for something for the rest of your life.
You say it's a "whole life" policy, which means it should have some cash value if you cancel it, i.e. you get SOME money back. Not everything you and/or your mother paid in, but something.