Easy enough solution is to have another timing mat about 100m from the finish.
It won't be a 'timing' mat as such, but will flag up whether or not the runner crossing it has crossed all the other timing mats; which to the cheats, have been placed in some very inconvenient places.
Needless to say, it will be relatively easy to drag possible miscreants from the race where their timing chip will be given a public examination.
If the chip proves faulty, then maybe a free entry for next years event as recompense for the potential public humiliation.
However, If the chip proves perfectly functional, a good public naming and shaming should be entertaining.
My feeling was that the online rivalry and discussion earned them a status above that of mere 'odd comment' posters. Fame of a sort.
I mean, there's 'household' fame as in the Farah sense, and then there's the famous in one's own household which is more common; but you can live in a place by yourself and only need a dog for that level, and then there's the sort of fame where 'someone knows you; for your activities and achievements, but you don't know them.
Needless to say, the last category doesn't include the sort of activities which draw the attention of the Metropolitan Police.