The best way to keep the religion clean would be to simply terminate the membership of unrepentant sinners, revoking their status as a JW so that they are now viewed as any other never-JW member of the public. JWs don't shun never-JWs. They just see them as bad association and are supposed to avoid having close friendships with them. There is even a little more leeway given if the never-JW is a flesh and blood relative.
Shunning is a bad way to keep the religion clean. Why? When members know the punishment for their secret sin is shunning, they just keep their sin secret and continue in it. So shunning deters many sinners from going to the elders with the result that there is much sin going on in secret within the congregations worldwide.
The act of shunning is not needed to keep the congregation clean from the defilement of unrepentant sinners. All that is required is that the person's membership in the religion be terminated. One of the greatest contradictions in JWs claim that shunning is needed to keep the congregation clean, is the fact that they allow never-JWs who are known practicing fornicators to participate in their meetings by commenting. So a failure to shun an expelled member tarnishes the congregation, but Jehovah is A-OK with a practicing fornicator participating in congregation meetings by commenting, just because he was never a JW? Really?!! Explain that logic to me. If they were genuinely concerned with keeping the congregation clean they would not be allowing non-JWs to comment at the meetings - especially non-JWs who they know are living immoral life-styles according to their teachings.
The real reason for shunning is control - controlling the members with fear and controlling (preventing) the flow of information from ex-members to current members. Shunning also serves to keep serious sins hidden.