This could be a tough situation to capture....my only thoughts are to still use the flash to freeze the dancer, but use a faster shutter speed to still give you the disco lights but it would have less effect on blurring the dancer.  Perhaps a manual flash set to a lower power (ie 1/8 power will give shorter flash duration) and use 1/2 sec shutter speed.  Let us know how you make out!

In youth, we learn....with age, we understand.