We wanted to cap a gas line that served a gas dryer (has shutoff and then extends to flare connection for flex line). Appliance was removed, shut off turn off. My son tried to remove the fitting to plug it but was unable to turn it. Instead of reefing to hard on the gas lines, he removed the flex line and then bought a flare cap and installed it on the fitting.

Is that sufficient for safety?

The line now has a shutoff valve in off position and then extends to the flare cap which was installed without pipe dope (ie flare).