RAW it always happens on death. I don't see a compelling reason to change that.
A high level Wiz/Sor could use a readied action to cast Resilient Sphere at the moment of death to contain the explosion, I think.
Opening the fight with a merciful disintegrate would deal with death throes, if your disintegrate is reliable enough (can beat the 20 touch AC, SR 31, and +29 fortitude save). Or you could slap merciful spell on a more reliable blast (Maybe a buffed up Snowball or similar), or make a merciful weapon for the front liner.
The point is that generally nonlethal damage is ideal when fighting creatures with death throes, because then you can detain them long enough to get everything valuable away from the blast zone. Remember, you only need some nonlethal, which means as long as one party member does nonlethal damage in a great capacity, the rest can do lethal all day. And that merciful spell has 0 spell level adjustments and the rod is the cheapest around. So buy the rod if you don't have time to take the feat.
Alternatively, Strategically placed walls of force can protect important locations to minimize collateral damage, Emergency Force Sphere can instantly protect you, Readied action resilient sphere can trap the blast as Plausible Pseudonym suggests, a successful offensive plane shift (Positive Energy Plane) can kill it where it can't harm you, Baleful Polymorph could theoretically make it safe to kill (probably won't though, unless you really pump your save DC's), Imprisonment will deal with the monster for a long, long time, luring it into an anti magic field will disable the ability, and mental block might disable it as well, though I'm not sure. Wizards have a big toolkit.