Daniel Udoh fired his side into a half time lead, but missed a golden chance in the second half.
Spartans were always in the game and Daniel Maguire got free to turn a cross home in the 89th minute.
Gavin Cowan admitted his huge frustration after the game, and didn't mince his words about his players failed to see out the game.
He said: "Frustrated to say the least, the lads should be kicking themselves and they will be if they have anything about them.
"Later in the second half we showed a lack of leadership, our strikers didn't seem to want to shoot today.
"We have five centre backs on the pitch and no one decides to pick up their striker.
"At any time that is criminal but in the last couple of minutes it is unforgivable."
It was the third game of the week for the Bucks, after the FA Trophy disappointment and a big win at Bradford Park Avenue in the week.
But Cowan made no excuses for his side failing to see the game out.
He added: "We have no excuses it is the same for all times.
"They had us under the cosh a bit and had come to get a draw or nick something at the end.
"We knew what they were about.
"If the lad scores from 30 yards out I'll clap him, but I can't have a number nine sliding in like that.
"It is criminal to do that."
The Bucks chief issued an almost rallying cry to his players, and insisted if they are serious about promotion they have to take responsibility.
He added: "If they are serious about getting promoted they have to cut that out.
"How many times has it happened?
"You look at Hereford 1-1 here, we were not ruthless and didn't show concentration.
"If we win them games we are probablt top of the league.
"They were superb in the first half, and in the second I put Jon Royle on to get the second balls and we changed it to plug gaps.
"Joe has made one save, and the lad has tapped in in the middle of the goal.
"It was a lack of professionalism."