Is there any command that makes fallen sand permanent?

You can force a FallingSand entity to ride an armor stand to prevent it from falling.

1.8:

/summon FallingSand ~ ~1 ~ {Time:1,Riding:{id:"ArmorStand",NoGravity:1,Invisible:1}}

1.9:

/summon ArmorStand ~ ~1 ~ {NoGravity:1,Invisible:1,Passengers:[{id:"FallingSand",Time:1}]}

However, FallingSand will not exist forever. Once its internal timer ticks to 600 while it's within the boundaries of the world, it will despawn. The preventative for that is to periodically set the Time tag back to 1, which will reset the internal timer.

/entitydata @e[type=FallingSand] {Time:1}

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Alternatively, you can summon an armor stand that is wearing the desired block on its head without the need for FallingSand, though it will be slightly smaller than a full block.

1.8:

/summon ArmorStand ~ ~1 ~ {NoGravity:1,Invisible:1,Equipment:[{},{},{},{},{id:"minecraft:sand"}]}

1.9:

/summon ArmorStand ~ ~1 ~ {NoGravity:1,Invisible:1,ArmorItems:[{},{},{},{id:"minecraft:sand"}]}

The method of spawning falling sand on top of an armor stand works great, to avoid the despawning of the falling sand-enity, simply set the age to a really big negative number. The maximum is -2147483648 or -2^31. but that should last you a good 107,374,212 seconds which corresponds to about 3.4 years of in-game time, if that isn't enough just reset the entity data tag with a command block.

This should work:

/summon ArmorStand ~ ~1 ~ {NoGravity:1,Invisible:1,Passengers[{id:"FallingSand",Time:-2147483648}]}

You can set a falling block timer to an extreme negative value. Because it ticks up by 1 every 1/20th of a second, it takes more than 3 years to vanish.

To prevent it from falling down, you can use the NoGravity tag.

So it ends up as:

/summon falling_block ~ ~ ~ {NoGravity:1b,Block:"minecraft:sand",Time:-2147483648}

"sand" can be replaced by any block ID.

/summon minecraft:falling_block ~ ~ ~ {Time:-2147483648,Block:Stone,NoGravity:1}

Okay, so this question is going on 3 years old, but I have come up with a working answer for it. In case you weren't aware, the first step is to enter the following command.

/summon falling_sand ~ ~ ~ {Time:1b,BlockState:{Name:"minecraft:stone"},NoGravity:1b}

Stone can be replaced with any block of your choosing. The next step, however, is what makes this permanent. Set up a command nearby the falling blocks (or do /execute positioned (their coordinates) run) that is repeating, always active and says the following:

execute as @e[type=falling_block,distance=0..5] run data merge entity @s {Time:1b}

What this does is constantly reset the time state of the falling block so it never reaches 600. The falling block will be permanent and look just like a normal block, but will be able to be passed through.

The best way I could find was to use the command:

/summon minecraft:falling_block <x> <y> <z> {BlockState:{Name:"minecraft:sand"},NoGravity:true,Time:-2147483648}

I have a slightly different method.

summon minecraft:falling_block <x> <y> <z> {BlockState:{Name:"minecraft:sand"},NoGravity:true}

Make that run every single tick, WITHOUT a Time tag, and as long as the command runs it will keep on replacing itself, and act exactly like a permanent falling block.

This only works if you're project allows you to do that though - if you need to execute commands as the falling block or something then use data merge to keep the time tag at one, as shown by Skylinerw.