Alright, doing a single removal like [ device name\Administrator ] will be pretty easy.
This does it well enough to knock out the one thing. (It has to do the cleaning in the two places: as its displayed, but also where its tested.)
SELECT DISTINCT A0.DISPLAYNAME AS "Device Name", A1.NAME AS "Name", replace(A1.MEMBERS,A0.DISPLAYNAME + '\Administrator;','') AS "Members"
FROM Computer A0 (nolock) LEFT OUTER JOIN LocalGroups A1 (nolock) ON A0.Computer_Idn = A1.Computer_Idn
WHERE (A1.NAME = N'Administrators') and replace(A1.MEMBERS,A0.DISPLAYNAME + '\Administrator;','') LIKE ('%' + A0.DISPLAYNAME + '\%')
ORDER BY A0.DISPLAYNAME
There's also one small issue with this, in that i'm matching on a string that includes a semi colon on the end. If the string is the only text or is the last entry, it won't have the semi colon, so it won't get filtered out. If you remove the semi colon from the query, it'll still work, but you'll end up with a less tidy appearance. Its up to you. Most of the time the built-in administrator account will usual be at the front on the list though, so it should generally always have the semi colon present, unless it's the only thing in the list.
The real problem will be that you are going to realize that there's going to be a whole list of items that you want to knock out more than likely, and what those things are will get progressively more challenging to make into a dynamic SQL query. It's probably going to end up being a function call that does all the scrubbing that you're going to want done and might have to be hard coded to achieve specifics in your environment. This would also be the place that tackles messy left over semi colons that might be left behind. Essentially, there should be a loop that parses the string and tests each entry against whatever evaluations that you want, and thus the reason for a function to do this part instead, in this case the SQL code would like something like this:
SELECT DISTINCT A0.DISPLAYNAME AS "Device Name", A1.NAME AS "Name", scrubusers(A1.MEMBERS) AS "Members"
FROM Computer A0 (nolock) LEFT OUTER JOIN LocalGroups A1 (nolock) ON A0.Computer_Idn = A1.Computer_Idn
WHERE (A1.NAME = N'Administrators') and scrubusers(A1.MEMBERS) LIKE ('%' + A0.DISPLAYNAME + '\%')
ORDER BY A0.DISPLAYNAME
and of course scrubusers(vchar) would be the special function that cleans the string up before its tested and displayed.
So now you need do is decide on that magic, and make that function! :-) And there you've slipped the slope! :-)