It depends on your use of English.
"Under the policy no member of the police service, whether police officer or police staff, may be a member of an organisation whose constitution, aims, objectives or pronouncements contradict the general duty to promote race equality.�
So if you said �are they banned from joining any political party�, I would say yes, the BNP, NF and IRA (perhaps others, I don�t know).
Or, you could say �are they banned from joining any political party�, and I could say no, except the ones that contravene the policy.