Hi Elie,
first of all you need to know what BGP communities are accepted by your ISP's.
You should ask them, because sometimes they do not publish all BGP communities .
You can use for example well known (if they are accepted):
- no-export - which means do not export to any eBGP neighbor
- no-advertise - which means, do not export to any BGP neighbor at all.--
Jarek