Find and destroy the Flesh That Hates spawner
Light up the surrounding area to reduce spawning
Replace nearby spawnable blocks with slabs, buttons, carpet, or glass
Cover the area with non-spawnable blocks
Increase the light level if the modpack uses light-based spawning
Move far enough away to unload the area
Use peaceful difficulty if acceptable
Disable mob spawning with game rules if allowed
Remove nearby dark spaces, caves, and tunnels
Use commands or admin tools to delete the spawner if you have permission