Well To stop the computers from getting stars all you have to do is:
Just Before Your Opponent Gets A Star Press Start And Go On The Control Setting Thing And Change The Computer Guy To A Player And You To A Computer. Then You (Now The Computer) Says No To The Star And Then Toad Goes Moooooooh! Now Press Start And Change Back So Its Like The Computer Said No And Was Really Stupid. This Also Works On Mario Party 2 .
When playing Adventure mode, you can take over any computer controlled character to stop him from buying or stealing your stars.
When you see a com. character close to a star, press START to pause, then press A to bring up the Settings menu, then bring up Controller Settings and change your character to com so when you reach Toad or Big Boo you can refuse stars or stealing coins. That way you can make your com player as hard as you want and still win.