Add 4 diodes, one across the contacts of each relay, to accept the flywheel current when you open the relays. Orientate the Cathode of the diodes to the positive rail.

If you look at it, you will find that a Bridge rectifier block, cunningly wired, will do what you need.

Put 100 Ohms and a 100nF capacitor in series, across the motor. Use a decent polyester cap, because you can be handling a fair current. I have seen little ceramics blown up by the transients

Steve