Scunthorpe themselves suffered an early blow when striker Paul Hayes was stretchered off and taken to hospital with a suspected fractured cheekbone after a clash of heads with defender Kyle McFadzean.
Alfreton did manage to frustrate Scunthorpe for half an hour until substitute Ben May headed the home side in front after a mistake by goalkeeper Tommy Evans, who failed to hold a shot from midfield man Grant McCann.
The League One side then took charge with Matthew Sparrow and Marcus Williams both firing shots just over before centre-back Krystian Pearce forced a good save from Evans.
Alfreton's hopes disappeared in the 48th minute when Clayton got his marching orders for an apparent off-the-ball elbow aimed at Scunthorpe captain Cliff Byrne.
But it was not until the 62nd minute when Scunthorpe struck again with leading marksman Gary Hooper scoring from close range for his 12th goal of the season.
It was one-way traffic as Byrne then struck the foot of a post for Scunthorpe and Evans saved well from both Martyn Woolford and May.
It was Hooper who struck again in the 84th minute from an acute angle after the Alfreton defence had been split wide open by a through ball from Sam Togwell.
It was not until the closing stages that the beleaguered visitors finally managed their first effort at goal with a looping head from substitute Danny Davidson providing the easiest of saves for goalkeeper Joe Murphy.
Scunthorpe wrapped up the scoring in stoppage time when Togwell drilled a low shot through a crowded box after Alfreton had failed to clear their lines from the last of a succession of corners from the home side.
