Moved provisionally to ePO for better support.
If you want to completely remove the machines from ePO's control, then deleting the machine and choosing the remove machine option will do the trick.
However if it's just the dashboard display, you could always edit the query that produces the dashboard display and exclude those particular machines from the results... just a thought.
One way to deal with this problem in terms of reporting is to create a decom group in my system tree. I adjust the dashboard queries to exclude the decom group. Then any time you decom a server, just move it into the decom group. Or better yet you can use tags instead of the system tree, tag a system as decom, and then exclude anything with the tag decom from your dashboard queries. Using tags allows you to leave the server in the original system tree location in case you have a multi-level OU structure you want to retain. Of course doing any of this means you have the possibility of a server that is brought back online but still isn't in the standard dashboard. I would suggest building some more checks and balances into this, like a server task that checks for a recent ASCI and removes the decom tag, or sends a report through email, or an extra dashboard query etc. (a million ways to not forget, just make sure you use one).
As JoeBidgood told you can delete these servers from ePO and if you don't want theam to appear again at the next AD sync then you can disable these computers under Active Directory; this way you don't delete them from your domain and ePO's sync won't see any disabled computer