Restricting a Few Users--Simplest Approach?

We have an existing subversion repository which uses an automated system
to generate an htpasswd authentication file from our domain controller;
all users in that file are given full access. I would like to modify
this setup so that a small subset of the users ("service" accounts that
are shared by various groups of people) are restricted to read-only
access. Is there a complement to require, something like a
"require-not"? The idea would be

<LimitExcept GET PROPFIND OPTIONS REPORT>
    Require-not service1, service2
</LimitExcept>

Thanks in advance for any help you can give.

-- 
Aaron Davies
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