I have Installed 10/3 wire for my baseboard heaters and have just found out that menards wire chart is wrong because I only need 12/2 or 12/3. My question is what do I do with the neutral wire since I don't need it. (Cut and tape it or what ) ?

With a straight 240 VAC you only need 2 conductor and a ground. Wire nut or tape both ends of the neutral and tuck back in the box. Don't cut since your wiring needs may change with another model.