Blasts Generally Can't be Defended Against; That's a Good Thing
Blasts are hard to defend against outside of using certain Spells, Rituals, or Proxies. Guns are also difficult to defend against, even if somewhat easier. The Dodge skill or a Bulletproof vest are generally the non-magical defenses against bullets, and they will rarely offer enough protection to characters. This is because characters in Unknown Armies are generally susceptible to injury just like real people, and whether a person gets hit by a Gun or a Blast is close to the same level of danger. Blasts, while harder to defend against, carry a fairly hefty opportunity cost to use; namely, being an adept and carrying the required charges. Any idiot can walk into a store and buy a Gun which can make someone just as dead as a Blast. The few examples of defenses against Blasts in official sourcebooks are practically non-existent; even high level NPC Alex Abel only has an artifact that provides retribution against Blasts. If you can't stand this lack of protection, consider creating a skill that works similarly to Dodge but works against Blasts instead of physical harm.
Long-Distance Blasts always do Minor Damage but Require a Significant Charge
On top of the usual cost, a Long Distance blast also requires a significant charge to send off. Minor blasts were compared to a really strong kick in terms of how much damage they do; imagine if, to kick someone somewhere else in the world, you needed to risk your life (or obtain a book worth $500 you hadn't owned before, or get a $1000 lump sum). If you really wanted this person dead, you would have to do this several times in the course of only a few days. Another important fact about Long-Distance blasts is that you must personally know and have spoken to the target for at least a few minutes; if a player character is getting hit by the blasts, it shouldn't take them terribly long to figure out who's blasting them. Unfortunately, there are no official artifacts or rituals to detect who has been blasting you, but they should be fairly easy to invent as a GM. It would even make for a good plotline: "Find the ritual/artifact to find who's been blasting you before the blasts kill you."
The fact that blasts are almost impossible to defend against is also one of the things that gives Proxy rituals their power; they're one of the few somewhat reliable ways of not getting hit by these blasts. It asks you the question, "Am I willing to let someone else suffer in my stead?" This seems to be intentional as is evidenced by another solid way of avoiding blasts; the Second channel of The Fool. The Second Channel of the Fool allows the Fool to randomly "bounce" damage they would have taken to someone else, friend or foe alike. Again, if you really don't like this, I'd recommend house-ruling in the Magic Dodge skill.
It appears that the general lack of defense against blasts is entirely intentional given the setting. In fact, written in the section on blast spells (Pg. 113 UA2), it says:
Most blasts can't be dodged.
In short: There's very few if any official ways to defend against blasts outside of Proxy Ritual like effects; this appears to be entirely intentional. Long Distance Blasts have no clear cut magical way to determine who it is that's attacking you, but the blasts are very unlikely to kill you straight away and you've at least spoken to the person who's blasting you, this gives you time to figure out who and find them. I hope this helps at least a little, and if this kind of thing bugs you, don't hesitate to add in spells, rituals, or artifacts that would help against this sort of thing.