There are various things you can try, although they may not all be feasible. If there has been actual dishonesty for personal gain, that would most likely be a criminal offence, and you'd need to involve the police.
If the bad behaviour is less extreme, then you can report councillors to the Monitoring Office - usually the senior legal officer at the district council. But the worst he can do is name and shame, and it can take a while. You may be able to raise a complaint with the council's auditor, but you would need specific financial issues to report.
The most practical approach is most often publicity and putting up alternative candidates for election. Public pressure can bring about resignations, and if you don't trust the council over co-option, 10 electors can demand a poll (although the parish will have to pay for it, it may still be worthwhile if the situation is bad). Otherwise, periodic elections will come round, and you need to have people ready to stand.