The result means Sam Wilkinson's team slip to 17th in the Evo-Stik South League after their second successive defeat in four days.
Injuries from Saturday's 3-0 loss at Romulus meant Kallum Smith and Luke Hornsey were side-lined along with striker Conner Robinson who will be out for another week.
New signing, Jack McGovern led the line in the hope of improving the Whites' mediocre total of just five goals in five league games.
The visitors started impressively and looked the brighter side in the first 10 minutes.
However that all changed when Nathan Whitehead hooked the ball over his shoulder beyond the reach of Whites' keeper Jake Turner to make it 1-0 to Spalding.
The Ashby Avenue outfit then responded with near-misses from Cotton and McGovern.
Ten minutes' before half-time there was more bad news on the injury front when young defender James Blunden hobbled off with an ankle problem.
Harry Millard moved to right back, Callum Ward slotted into central defence and substitute Jack Wightwick went out wide.
United's next chance came just after the break when the ball was scrambled off the hosts' line.
Five minutes' later, former Whites' front man, Gary King rifled the ball home with a strong left footer to double his side's lead.
After that, things started to get interesting.
Spalding's Conor Marshall was sent off in the 65th minute for dangerous play. Ryan Oliver's shot was then brilliantly saved by Duggan in the Spalding goal.
Whites' skipper Phil Watt was shown yellow in the 75th minute but then Harry Millard's magnificent run down the right set up Rob Norris who pulled a goal back.
In the 80th minute, Spalding substitute Jonny Lockie saw red for violent conduct leaving his side with only nine men on the pitch.
United poured pressure on the opposition defence and had a justified claim for a penalty turned down by the referee.
Sam Wilkinson's men had opportunities to return with at least a point but it was the same old story. Chances were not converted.
Perhaps when McGovern and Robinson resume their successful partnership from last year, goals will come more frequently.
Peter Hayward
# | Team | Pl | Pts |
6. | Stamford | 42 | 71 |
7. | Leek Town | 42 | 68 |
8. | Lincoln United | 42 | 68 |
9. | Corby Town | 42 | 60 |
10. | Cleethorpes Town | 42 | 55 |
11. | Stocksbridge Park Steels | 42 | 53 |
12. | Peterborough Sports | 42 | 51 |