Institute an application process which weeds out the hoppers and kick out new recruits prior to wars until they have proven themselves.
SC is not going to institute a penalty or punishment that coerces people into staying in your clan and participating in your war. People have free will and leave for a variety of reasons, many of which are not deserving of punishment.
Even if they did do that, these members who are now forced to stay would most certainly undermine your war effort anyway. Did this not occur to you?

