Another possibility would be starting an activiity that blocks the step counting. It may depend on the model of your watch, whether some ativities do it or not - my Instinct does not count steps while in any kind of cycling or swimming activity. So you could for example start an Indoor Cycling activity (it does not use GPS, hence it won't drain the battery), and discarding it at the end. Unless you want to keep it as a log of the driving - in that case you could even create a custom activity from a copy of the Indoor Cycling, and change the type to Driving later in Garmin Connect.
Some watches probably offer the Driving activity directly (I do not have it on Instinct, though). And for those that do not have it, the custom activity FIT file could be hacked to report the Driving type directly.
