TL;DR: Surround it with non-flammable blocks.
Lava has this to say about firespread:
In order for air above lava to turn to fire, a block adjacent to the air has to be flammable, or one of the wood-constructed non-flammable blocks. Since catching fire depends on air blocks, even torches or lava itself can prevent a flammable block from catching fire. Additionally, not all flammable or wood-constructed blocks can be ignited by lava.
I set up a Control and a Test for this:
Obviously, The Control set fire pretty quickly, however, the Test is yet to show any signs of catching fires.
On top of this, the wiki states that lava also needs to be a Source Block to cause fires. if it is flowing, it won't.
Lava of any depth can start fires this way, whether or not it appears to have a current, but it must have settled into a stable
lava block. The
flowing_lava block will not cause fires.
However, after a little testing, I am yet to re-create this. I have done it with and without source blocks (Placed the lava, then picked it up again after it started flowing), as well as with various different setups (vertical, diagonal, and horizontal), but all set fire to the blocks.