How can I disable a silverfish spawner?

Solution 1:

They actually stop spawning at light level 12 or higher, so surrounding the area with glowstone or redstone lamps should work.

Etho, a relatively famous YouTuber who specializes in exotic mob farms, has been working on a silverfish farm for a while: he went over how to disable a silverfish spawner using only light in episode 155 of Etho Plays Minecraft:

He had the same scenario you have: he wanted to keep the portal accessible but disable the spawner. He goes into possible caveats and problems you might run into, but he was able to completely disable the spawner by adding redstone lamps across the top of the spawning area.

Solution 2:

If you want to deactivate a silverfish spawner, it is simplicity once you know it spawns in a 9x9x3 area and requires the light to be above 12. Remember that the portal blocks are modeled after glass and are transparent. This configuration requires 13 JackOLanterns, 3 torches, and some dirt.

Also, it appears that the lava is positioned to where if it were completely released from its nicely contained reservoirs by removing the stairway and pool walls, it should almost light up the room sufficiently, at which point one may want a glass walkway above it.

Happy Crafting