In book on the Soviet SAM tactics in Vietnam, they describe the following methods to counter these deception techniques by S-75 operators:

Early self-protection jammers needed a 4 second delay to tune-in in the signal PRF. This could be exploited in two ways:
1) cycle 75km/150km switch: false targets jump in range, while true target remains on the same range mark.
2) turn ant/ekv off/on: true target dissapears, while false targets remain.
3) IIRC, S-75 also had a variable PRF ("Peremennaja Chastotota") mode, which could make false targets drift in range and that would be noticed by operators.
4) If everything else fails: an experienced operator can distinguish true/false targets by their form, intensity and any present artifacts (target marks of sharper squared shapes and different thickness); true target becomes brighter than false targets on approach, or will fade faster on receding than the false imitations).

1) for early generation Dvina and Volkhov, there was no technical method to counter angle jamming, except to use manual guidance. The closer is the target, the smaller is the tracking error due to the better return vs noise ration. My guess would be ~15km is a burn-through range, approaching/receding target with P=0 were angle jammer completely loses any effectiveness (warning: I did no calculations though)...
2) Volkhov had LORO mode, which made it much less susceptible to angle jamming because the scan rate information was no longer available: without giving the attacker scan frequency ant main beam time on target, the angle tracking error is reduced.
3) In later generation S-75 Volkhov and S-125 Neva, the GshV and GshN modes were introduced. It provided the filtering of angle jamming and sidelobe blanking, reducing angle errors further or negating it completely at much longer ranges.
Unfortunately, GSh modes were implemented in a very cheap way, and they were NOT compatible with SDC against low flying targets or targets in chaff clouds... Only S-25 Berkut defending Moscow had a fully capable and more expensive GSh system.

1) Early gen. systems: manual tracking in range.
2) In GshV and GshN modes, S-75 and S-125 had a built-in capability to track on leading edge only in range (I've seen this mentioned a couple of times in GShN and GShV design and field test bulletins).