Spent where?
As of November of 2009, only $100 billion of that $787 billion stimulus was actually spent with another $90 billion of stimulus coming in the form of tax reductions = $190 billion.
http://cbo.gov/ftpdocs/106xx/doc10682/11-30-ARRA.pdf
The bill was specific as to what things the money would be spent on. The money not spent yet is for stuff that hasn’t been started yet or paid yet. Those things will still be done and the money spent. The extra $75B comes from stuff they added to it later.
They would have to pass legislation that takes $75B away from something in the original bill in order to not over spend, but have not done that.