That is a good question.
It is extremely difficult to establish a causal relationship between financial irregularities and the outcome of the election. It's difficult even when it comes to spending. Campaign spending sometimes exceeds the set limit. Yet, it is extremely difficult to draw a conclusion on how overspending may have impacted the results in a particular riding.
There is, however, a mechanism for this: contesting the election. If someone believes that the election results were influenced, there is a mechanism for deciding on the issue.