i am setting up new blackberry server with exchange 2007 but while we were configuring the Besadmin account and its permission; we execute the below command (as mentioned in blackberry documentation) getmailboxserver "<messaging_server_name>" | add-adpermission -user "BESAdmin" -accessrights ExtendedRight - extendedrights Receive-As, ms-Exch-Store-Admin. and now besadmin account has right on every mailbox but i want to give besadmin permissions on specific accounts (mailboxes which will use blackberry) so how we can do that?

DId you follow everything here? http://www.blackberry.com/btsc/search.do?cmd=displayKC&docType=kc&externalId=KB02276 Assign permissions for a BlackBerry Enterprise Server service account Otherwise, Im not sure what you are asking.

Go to each and every mail user who is a Blackberry user, and grant the BES admin account "Send As" rights. Either use the ADUC, or the Exchange management console. Maybe create a separate OU in AD just for your BES users, and then set the Send As permissions on the OU level.

but now i want to cancel this command: getmailboxserver "<messaging_server_name>" | add-adpermission -user "BESAdmin" -accessrights ExtendedRight -extendedrights Receive-As, ms-Exch-Store-Admin how can i undo or cancel the above command?

Use remove-adpermission to remove permissions.