Unless I have missed it... debt does not stop you rejoining a nation.. more than 75 hostility stops you and hostility penalty ends once you drop the flag and fades away hourly after that. Once a citizen again then debt will cause the hostility penalty to resume nut only after the next reset. So, plenty of opportunity for the returning citizen to clear their debts before hostility hits 75 and kicks them out again. Even then judicious use of voodoo can delay that point indefinitely.
If the nation feels that the debt should be excused it can vote a pay-out to re-imburse the citizen after they clear it themselves.
I'll agree that having a vote to forgive a debt works too but really whats needed is a mechanism to enforce debts not another way to avoid paying them.