How to properly snub transient spikes from a transformer and prevent switch mode power supply burn out

I've received a failed switch-mode DC-DC step down converter (part# LTC3631 from Linear). This circuit was driven by a 24VAC transformer which is rectified on the circuit:

^{simulate this circuit – Schematic created using CircuitLab}

The leads from the transformer to the circuit are quite long (>5m). Apparently the 110V side of the transformer was switched off somewhat frequently lately which likely caused a spike on the secondary (24V) side. The input pin of the LTC3631 has a visible mark where the blowout occured (gas escaped).

What's the proper way of preventing this from happening again?

this question is possibly related.