How to stop my child from taking his middle finger out?

My advice would be to be more clear. "taking his middle finger out" means nothing unless he has it in something. You might try "displaying" or "flashing" or showing or any number of other verbs, but "taking it out" means nothing to me.

It sounds like there is no consequence for his actions away from you, seems like the school and the coach are not taking any action to correct the behavior,

don't make a big deal out of it, you told him once, say no more...[ you will just make it worse ]....he will have to decide for himself, and he will.....[ I'd tell them goatsmelling teachers to get over it,] he's only 7, and can't be held accountable for his actions, want to see fireworks, complain to the police the teachers are harassing a boy of only 7yo, not yet accountable for his actions, but they wish to force their will on him, oppress him....[ I think that will shut them up ].......

when punishing is not enough, it's time for spanking. they need to understand, it's not because you hate them, it's because you hate what they are doing. a little pain goes a long way. you don't have to injure him, just slap enough that it hurts. the fear of pain is a great motivator...

Cut it off. Both of them.

i will take some of his favorite things away like his skates and scooter and any devices as well. i have alreay spanked him a few times it seems to have worked at the moment he cries it out then he will come to me and say im sorry mama i wont do it again i luv u...i of course believe him but now its just getting out of hand...some teachers have seem him do it but its always the he did it to some child and the child told on him to the teacher and then i go to asking him if its true he will cry bloody marry telling me that its not true so what do i believe my child of course. i will see how it goes when i take his things away...thank u so much to those that have answered...

Has the teacher actually SEEN this action for themselves or are they just taking the word of the other students.

There is always a chance that your child has stopped doing it but that now there are some other students that are being little bullies and trying to get your child in trouble since they know they can.

If a teacher has actually SEEN your child do this, then take away everything. Leave the bed and clothes in the child's room and take away everything else. Tell the child that they only get the toys and electronics back when you have at least two weeks of no bad finger reports. If you don't want to go to that extreme right away, then just take away a few favorite items.

Sure, you have already tried not buying more toys - but if you start taking away the things the child already has until they don't have any of their favorite things anymore - that usually works. It worked for my children.

Let him suffer the consequences. And make sure no family members or friends are in the habit. If it were me I would have another talk with him and after emphasizing how inappropriately he is acting, work with him to decide what the consequences at home would be.

You're not going to like this. I'd spank his bottom.

The school needs stronger discipline measures, I think...