Bill Zastrow fears he’s about to lose $240,000 he put into General Motors bonds to finance his kids’ educations, and he puts lots of blame on one man: Barack Obama. “GM bondholders are not speculators as President Obama calls us,” said Zastrow, the 58-year-old president of Millbury’s FileMark Corp. “We’re talking about people’s retirement funds and education funds. These aren’t derivatives or anything like that.” As GM approaches a government-mandated June 1 deadline to either reorganize or file for bankruptcy, Zastrow and other small investors have formed “GM Main Street Bondholders” to call attention to their plight.