Couple a Random Number Generator to the LIDAR generator & modulate the pulses as per the RNG.
The LIDAR sensor ignores everything not on that exact modulation as obviously not having originated from itself.
Every RNG seconds the RNG+LIDAR sequence is reset, recalculated with a new RNG, and thus the filtered signals are never the same from cycle to cycle.
In order to spoof such a system one would only be able to do so for the duration of a single cycle, at which point the targeted LIDAR unit picks a new RNG, filters out everything not modulated TO that RNG, and thus renders the spoofing device moot.
The spoofer can scan the target again & try to spoof the results, but it can only do it for however long the target is still using the detected RNG sequence.
The moment the target switches RNG patterns, the spoofer has to start all over again.
Granted if enough vehicles on the road are using the same RNG+LIDAR sequencer system, the spoofer is bound to find a number of targets using the RNG that will then react to the spoof, but that only lasts as long as the targets are still using that same RNG sequence.
TL;DR: Connect a random number generator to the LIDAR pulse generator, have the LIDAR sensor ignore everything not modulated to that exact RNG sequence, and change RNG sequences every random seconds.