[Revoke] cannot revoke the permission of "some User"....

Hello all,

when i use the "\dp" to view the permission of "mytable":

mytable | {"=","28=arw","34=arw","36=arw","40=arw","42=arw","cgi=arw"}

i guess those numeric thingy is kinda object id of a user, and i've deleted
the user b4 i revoke the permission of that user from "mytable".

is it possible for me to remove those 28, 34 & 36?

thanks in advance.

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matthew