Radulf St. Germaine said:

What do you think they have Fate Points for?

Something that could be used once or maybe twice is a psyker with the Endure Flames minor psychic power, Pg. 27 Disciples of the Dark Gods. They'd walk through the flames with no problem!

Oh and have the same psyker use spasm on the guy with the flamer to target his own guys if you really want to be mean.

Also there is that Pyromancy power called Douse Flames and while in the area of effect of the power fire ceases to function in till the power is stopped or the psyker leaves the area.

One of my players has a Cleric/Heirophant that has a nice custom-built Combi-Flamer/Melta so she doesn't have to decide which way to burn heretics in advance.... Absolutely NASTY in close-in fights!

So the team is investigating a shot up convoy in the middle of a town square (not especially open terrain, but there are hab-blocks separated by about 50-70m of cobblestones and assorted topiaries) and some PDF troopers misdirected by a rival faction into thinking the team are heretics (someone with a badge said so!) take up positions in the surrounding buildings and open fire!

Everyone dives for what cover they can find (park benches, nearby vehicles, etc.) and those with long ranged weapons begin cautiously scanning for targets. The Magos turns on his force field and takes flight with maglevs in the hope of flushing out the shooters (so he can then drop on them with a huge axe!). Meanwhile the Cleric.... sat there behind cover and muttered to herself.

The moral of the story: Different weapons for different situations. Flamers are absolutely useless at anything more than 20m. The basic generic autogun or lasgun can engage out to 400m if you have line of sight. Sniper weapons as well as most support weapons can engage at much greater ranges than this. Mix up your encounters some. I have found that I can get more variety in mapped battlefields if I sometimes use a 1" = 2m scale instead of 1" = 1m (and inform my players of the map scale!). Smaller than this and movement becomes almost pointless. If I need an even bigger fight scene then I will either map part of it or simply go into narrative combat. Even the main hall of a modest Imperial church is a fairly large scene to stage a combat in, let alone a factory floor or a cathedral.

Scenery can also dictate wisdom. Do you really want to use flamers when your suspect is feverishly researching something in an Administratum archive vault? How about when you finally corner the infamous "Big Bommah" who has been "purifying" important structures within the hive in a misguided tribute to "Tha Emprah wot talks to me in me 'ed!" (Energy criticals on a guy with lots of high-yield explosives! YUMMY!!!)?

I would use flammable liquid in bottles, like alcohol that just lights as they use their flamers.

I know that acolytes have some measure of the Inquisition's ability to wave two fingers at the rest of the Imperium's rules but if the group's overuse of flamers in so endemic somebody important should probably have taken exception at them setting fire to everything by now.

Most plausably the Ecclesiachry, after all setting fire to church buildings (even accidentally) woudn't exactly endear you to them. Nor are they the most reasonable people to try to calm down when they decide to do something about it..