L2J Server

I want to stop seven signs at all and stop almost all the castle sieges excepting Gludin Castle . Can someone tell me how ? I mean i know that i have to close it from core but i don't know from where and what to delete.

remove the associated npc's

yes but they will still work and use ram

If no one registers for a siege (i.e.L the registration NPCs are not there), there will be no siege, therefore it will not use ram.
The only thing that will still work in the background is the countdown to siege time and then the check to see if clans have registered. However, unless you're running your server on a 1990's PC, it will not make any difference in performance.
Dumpster's suggestion is really the easiest and will work fine.

If after doing this you still have problems with RAM usage, then you need to consider a few things:
1. Are there custom scripts that you use which could be causing a memory leak? (i.e.: poorly coded scripts are very common in the "custom scripts" section, so this would not be surprising)
2. Should you restart your server a bit more often?
3. Should you get a server with more RAM?

The think with Castles is not only for ram , it is actually an idea where clans have to fight only for one castle , it will be only a main town with some farm zones nearby. So we will need only one Castle , no Instances , no Clan Halls. It is just an idea for a PvP server.

hmmm... so?
The castles, clan halls, etc... are in the client, you cannot remove them