Is it normal to prevent someone from coming to your funeral?

Not sure it matters once you're dead, but you can always ask friends and family members to help you with that.

fake your own death and observe your funeral from a distance at first. If this person shows up, nonchalantly walk up to them at the grave side and say to them "I thought I told you not to turn up...!"
That's the last time they'll turn up to your funeral!

Kill them first?

When you make funeral arrangements for yourself
just tell the Funeral director that you just want
a private viewing with no visitation or immediate family
only can attend.

Who are you wanting to prevent?

It is normal, unless you are being unfair. But it is ultimately your choice anyway.
Just declare that they are unwelcome in your will.

You can tell them up front, if they respect you, they won't show up. Otherwise you can get your family to bar them/escort them away if they show.

I understand the want to, sometimes there are family feuds and other drama that you just don't want at a funeral, more for the sake of your family than yourself, as I learned last year at my Dad's funeral...

Anyways I hope you haven't had any bad news? I'm sorry if things are actually heading that way, it just seemed an odd question at ask if you're healthy.

I don't think you can.

Send them a letter to tell them they are not welcome. Hire security protection for the event. Security is often hired for funerals to slit the throats of unwanted guests, especially by Italian families in New York City.

Don't die.