Thanks for explaining. Now I understand.
The resistor is the only way to adjust that.
You could use an programmable resistor for the load resistor, so the Arduino could set the sensitivy by itself. That way you can control it all in software.
Instead of a programmable resistor, you could even use a few resistors connected to Arduino outputs. By setting those outputs either to high impedance (set as input) or ground (set as output LOW), you have a programmable resistor.
A hard squelch seems to me not possible, it is almost impossible to decide when a gas is detected. Using a tone (like in a metal detector) that increases frequency with a higher gas concentration should work.
The bars on the LCD could use some automatic scaling or a logarithmic scale.
Do you use a fan to pull air into the sensor ?
If you sweep the sensor, the air flow is not constant, causing inaccurate readings since the sensor is one moment more cooled by the air than at an other moment.