Sorry --- as distasteful and morally wrong as their actions were --- it was not voter fraud. They did nothing illegal.
In Mississippi, it is an open primary, meaning anyone - regardless of affiliation - can vote. The Republican party is the one who decides that for their primary. They simply need to change the rules. Same thing happened in Colorado a couple years ago, and we changed the rules. Problem solved.
As for the actions surrounding the voter rolls, THAT is worthy of investigation. If it's found true, then somebody should hang (probably shouldn't say that in Mississippi, huh?)