Details

Description
-essentials.spawnerconvert.* does not prevent disabling this function
https://github.com/EssentialsX/Essentials/blob/2.x/Essentials/src/com/earth2me/essentials/EssentialsBlockListener.java lines 36:48

Steps to reproduce
Add -essentials.spawnerconvert.* to your user directly or group
Place a spawner
Use a mobegg on the spawner

Expected behavior
I expect it to not change a spawner if it is negative

Screenshots

I updated my response to your question in the MOSS, but I'll put it here as well:

This permission is not for the egg spawner placement, sorry for saying that it was. It's actually checking if you have permission to place the spawner, from what I gather.

Essentials doesn't handle egg spawner conversion, it is a vanilla MC thing (you can confirm this by going into a SP world and doing it). You'll need another plugin to block this kind of action.

essentials.spawnerconvert.<type> is a compatibility feature for certain older server implementations to ensure that stacks of spawners that were spawned in using /give have their types applied properly when you place them. If you read the code you linked, you'll notice that EssentialsX doesn't actually do anything unless you have this permission.

As @Kadenscott noted, EssentialsX doesn't handle changing the type of spawners using spawn eggs. You'll need another plugin to prevent that type of behaviour.