Getting rid of ghosts?

Hi all,
I took this picture of a friend's son playing with a sparkler. She really likes it but I would like to know which settings to use to get rid of that "ghosting" effect. I used the onboard flash as I couldn't be bothered getting out the SB800 and wasn't sure which mode to use.
ISO 200
Inbuilt flash on rear curtain slow.
1/2 second
f 7.1
Thanks,
Marvin
I took this picture of a friend's son playing with a sparkler. She really likes it but I would like to know which settings to use to get rid of that "ghosting" effect. I used the onboard flash as I couldn't be bothered getting out the SB800 and wasn't sure which mode to use.

ISO 200
Inbuilt flash on rear curtain slow.
1/2 second
f 7.1
Thanks,
Marvin