How can you legally keep your family from having a funeral for you?

If you are under the age of 18, there is probably nothing you can do. If you are 18, you can make another person, who is not a family member, your power of attorney and make out a living will and a health care directive with your choices noted. You must have these directives notarized (go to a bank and they will do this). Give your family and the person you have chosen a copy of these papers so they will know your wishes. Unless you are living with the person you are designating as your power of attorney how are they going to know you are dead?
Also, if you are thinking about hastening your demise PLEASE consider donating your perfectly good organs and other body parts to someone who is dying because their organs do not work anymore. (My brother needs a liver.) Someone should benefit from those healthy organs. Be sure and put this in your legal papers and NO ONE will be releasing your body to anyone because it will be going to many different people who will be extremely grateful that you were so thoughtful. Please designate your liver to my brother.

create a will and have it notarized.

You should really speak to an attorney about that, and about arranging your estate handling. Whoever you have agreed to have as the executor of your will would handle your remains in accordance with your instructions.

draw up a will, but your request in that document.

That depends on the law where you live, it varies from country to country, even within the districts of some countries…I would ask your local legal aid and if it is possible where you live they can tell you what steps you need to take.

Well if you told your family, I’m sure they’d wanna respect your wishes and not have a funeral..

Please do not commit suicide.
Please read this first,
http://www.metanoia.org/suicide/
Suicide is a permanent option to a temporary problem.
And that problem can be fixed. Please do NOT commit suicide.

Each state may have laws on this. I suggest you consult a lawyer and get a will put together. You could arrange and
pay for your own funeral in advance. Pay for your own plot
and leave your wishes with an attorney. Hope this helps.

You can’t prevent them from having a funeral. Don’t worry, you don’t have to go to it. A funeral is a gathering (often religious) during which people share memories of the person who died.

Putting it in your will is a waste of time. You cannot prevent people from congregating after your death. And a will is generally read long after funeral arrangements are done and over with.

The disposition of your body, however, is a different matter. Yes, you should check with a lawyer. Anybody could be named as an executor of your will and responsible for carrying out your final wishes, including making decisions on the disposition of your remains. If you choose to dontate your body (or your organs) to science, for instance, your executor has the legal right to ensure that this is done despite the family’s wishes.

See a lawyer.