I tried out various things
- Railcraft fire boxes
- Furnaces
- Gregtech machine casings with lava in the middle...
- Thermal Expansion tanks/liquiducts filled with lava.
- Warded Jars filled with ignis.
- Factorization Heater blocks.
- Alembic filled with ignis
- and various other things.
Nothing I tried worked so I guess just insulate the area with a non flammable block that you like to prevent spread.
...a fire (or lava) block can turn any air block that is adjacent to a flammable block into a fire block. This can happen at a distance of up to one block downwards, one block sideways (including diagonals), and four blocks upwards of the original fire or lava source.