Refs tend to not like sending players off because they get slated for ruining games.
How about, the offending player has to be substituted and cannot return?
That way the offended team gets some benefit from his actions.

How about, graded penalties like in something like ice hockey.

If its worth a yellow straight yellow same for red, not sure if its just a penalty or a sin bin, a new short sin bin, say 2/3 mins.

a 5 minute sin bin if your put on report and any subsequent ban, 1 game is against the team the offense was against, Ie a 3 game ban, miss next 2 games, 3rd game against opponents, whenever you play them again

But what if said offending player joins a new team before his current team play the offended team? Sorry it doesnt work because of this problem.
Best thing that can happen is an automatic yellow card is given for any offence commited that merits an on report andif the ref is unsure who the offending player is then its automatically the captain (or acting captain if the captain is off the field) who cops the 10 mins. this I think would also get the captains on the pitch really making sure his side are not commiting any foul plays.

Trouble is, on report occurs when the ref hasn't had a good view (or when he's being a bottle-less get). What happens in the former instance if there hasn't actually been a serious offence committed?

Bring back the biff!