Is there any block that Minecraft dispensers can shoot through, but makes the dispenser unreachable by players?
Okay, here's a list I came up with. I tried almost all blocks. Boldened blocks are better to use:
Paintings (Although arrows will kill them)
Ladders (placed on the dispenser)
Double Tall Grass
A Fence Door placed so that it opens when the dispenser is powered
A (Iron or Wooden) Door placed so that it opens when the dispenser is powered
An Iron Trapdoor (not Wood!)
And that's it. There is most probably no other block in Minecraft that can do that.