Be nice to actually see this before ruling. But from the description:
That’s not unsportsmanlike conduct. UNS at the high school level is a non-contact foul. If there’s contact, it can be a personal foul, but not UNS.
If it were 2 blocks in the back during the same live ball, only one can be enforced.
The block in the back is a “post-scrimmage kick” foul. Ball belongs to R. 10 yd. penalty enforced from the end of the kick, spot of the foul, or end of the return, whichever is more favorable for Team K.
If this actually were a UNS foul, it would still be R’s ball. The 15 yd. penalty would be enforced from the succeeding spot, where it would be 1st & 10 for R.