I've suggested this before & I guess it's fallen on deaf ears. If you want to stop the sandbaggers the only way to do it is to put in numerous methods to boost someone's rank/ tier level. If you never play any game modes that reward credits (aside from the weekly/ monthly free for everyone tourneys) then who cares, no need to mess with those people.
The second someone decides to step into the playing for credits arena, they need to fall under a new set of rules/ ranking.
-Win percentage too high against superior or equal tiers, guess what, move them up!
-Average too low in READY GO's (not ranked rounds), guess what, move them up!
-Achieve a certain milestone of birdies or eagles, move them up!
The only sandbaggers that are gaining a true advantage are those who play MPC's & Ready Go's. Don't sweat the details of messing with the people who can't or wont play for financial reward, FOCUS ON THE GROUP THAT ACTUALLY HAS SOMETHING TO GAIN FROM SANDBAGGING. It's not rocket science & by and large this type set up would only burn those who choose to abuse the system for personal gain.