Hi Elladin,
I am not too sure how to disable siege, as I am only beginning to self teach myself modding, but I have increased the cost of siege so much that they are never created - which results in the same outcome.
Each siege unit has its own .ini file in the edain mod, so there are several .ini files you must alter. I will list them below for you to make it easier for you to copy and paste to search/find/alter. I changed the command limit of all siege units to 90000 so they cannot be made.
GONDOR SIEGE
data\ini\object\goodfaction\units\men\trebuchet.ini
data\ini\object\goodfaction\units\gondor\gondorbatteringram.ini
ROHAN SIEGE
data\ini\object\goodfaction\units\rohan\rohantrebuchet.ini
data\ini\object\goodfaction\units\rohan\rohanbatteringram.ini
LORIEN SIEGE
data\ini\object\goodfaction\units\lothlorien\lothlorienflinkbaum.ini
data\ini\object\goodfaction\units\lothlorien\lothlorienbaumbard.ini (Treebeard)
data\ini\object\goodfaction\units\ents\entsinfantry.ini
IMLADRIS SIEGE
data\ini\object\goodfaction\units\imladris\dunedainbatteringram.ini
data\ini\object\goodfaction\units\imladris\imladriscatapult.ini
DWARVES SIEGE
data\ini\object\goodfaction\units\dwarven\catapult.ini
data\ini\object\goodfaction\units\dwarven\dwarvenram.ini (Demolisher)
MORDOR SIEGE
data\ini\object\evilfaction\units\mordor\catapult.ini
data\ini\object\evilfaction\units\mordor\mordortrollcatapult.ini
ISENGARD SIEGE
data\ini\object\evilfaction\units\isengard\batteringram.ini
data\ini\object\evilfaction\units\isengard\ballista.ini
ANGMAR SIEGE
data\ini\object\evilfaction\units\angmar\batteringram.ini
data\ini\object\evilfaction\units\angmar\angmartrollsling.ini
Just go to each .ini file I have listed above, type Ctrl + F, which will bring up the search bar, and type in 'CommandPoints' then hit enter. Each time you hit enter, it will automatically take you to any section that lists 'CommandPoints'. Adjust the amount to something high, like 90000 and neither the AI or yourself will be able to make them. (I did try changing the cost of resources, and found the AI would save up for it - so best to just change the commandpoint cost instead).
Example: CommandPoints = 90000 ;COMMAND_POINTS_CATAPULT
Hope this helps you! :-)