Get the youngster to imagine surrounding themselves in a bubble of white light for psychic protection which will help them to be more in control of their gifts. If they can't visualise the bubble of light, they can imagine a lovely white fluffy cloud coming down from the sky and just stepping inside it.
If the visualisation is still a problem, get them to draw a picture of themself and then draw a circle all the way around themself in the picture, then colour the inside of the bubble, white or pale pink. Or put a fluffy cloud around him or her in the drawing.
If the youngster sees spirit and is uncomfortable, they can simply tell it to go away. If it doesn't go away, they should then imagine a white light bubble around the 'spirit' and then tell it to go to the light. Then they can blow at the image as if blowing it away, like you would blow a soap bubble. Then tell it to go away to the light.
If there are problems with this visualisation they could always try the drawing method again. Draw what they think the image looks like to them and when it is drawn, surround it with a light bubble and tell it to leave and go to the light. If they are more comfortable with prayers, they can say a prayer and tell the spirit to go to Gods lovely light in heaven.
If the youngster can remember to put the white light around themself every day, this will help. However, it is more likely that they youngster will simply forget to do this, and then I would only remind them to do this when they starts to 'see' things.
They can also ask for loved ones in spirit to come and help. This could be a member of their family who is now in spirit, guardian angels, spirit guides, whoever they feel comfortable with. As their helper, they will guide and protect them and will help remove the spirit who needs assistance.
Remember too, particular with teenage children, that Ouija boards are not toys. They can, and do, open doorways to the spirit world that allow spirits from the lower planes to walk right into your living room. It is akin to leaving your front door open and allowing anyone to walk through the door. You would have no idea who those people are, or what sort of personality they have, and so extreme caution is needed. If the Ouija board is to be used, it is essential that there is someone present, like a Medium, who knows how to close those doorways down after use.
Above all, let your psychic youngster know that you don't think they are being silly, and that they can openly talk to you about any fears, concerns or questions that they may have regarding this natural gift that we are all born with.