The Hoops boss has received death threats and requires 24-hour surveillance in Glasgow and then on Wednesday night in Edinburgh, he was attacked by a fan during the clash with Heart of Midlothian.
The incident has created massive headlines in the UK and Spencer, who is a former Rangers and Scotland player, says it is hard to believe that this type of behavior is occurring in this day and age.
"It's embarrassing for Scottish football to be honest with you, and Scottish people in general," Spencer told MLSsoccer.com's ExtraTime Radio podcast.
"It's described as a dark day [in Scottish soccer]. I totally agree with that multiplied by 100. It's absolutely embarrassing that someone has actually taken it so far.
"I don't know why Neil Lennon is so hated in Scottish football. He was a great player for Celtic and won a lot of trophies there. That's crossing the line and it should never, never happen in the world of sports."
And Spencer wished for the sectarianism between religions in Scotland to stop as it only causes trouble.
"I don't know how it's going to stop and I don't know how we stop this bigotry and the mentality. It's a sport."