@Amerion (and others) - would something like Section 5 (below) alleviate your concerns about allowing an appeal?

It would give the government seven days after a successful appeal to make a new proscription, if they felt it was warranted. Would this provide sufficient time to rework the original proscription?

Proposed amendments to the Proscription Act Wrote:4. Judicial Review

(1) Individuals, regions or organizations subject to a proscription may challenge the issuing authority's determination of hostility in the High Court.

(2) Individuals who can demonstrate they have been directly affected by the proscription of a region or organization may challenge the issuing authority's determination of hostility in the High Court.

(2)(3) Individuals subject to a proscription of a region or organization may challenge the issuing authority's determination of their membership in that region or organization in the High Court. 

(3)(4) For the purposes above, the individual or an individual representing the region or organization must be granted adequate permissions to participate in the High Court proceedings for that case.

(5) Should the High Court rule that a proscription or its usage is invalid, the ruling shall not take effect until seven days after it has been delivered.