I have 2 baseboard heaters wired to a double pole 30 amp breaker. I am eliminating one heater and caping off the wires . Is this OK? I measure 120 volts at the white wire and groung, is this 220 volt wiring?

Yes it is 240 volts, and yes capping unused wires is a must, and the wirenuts and exposed wire with no sheathing must be in a junction box with proper cable connector and a blank cover.