He told his players they had no right to a Third Division championship medal, and said his comments behind closed dressing room doors would not be the last they hear of the issue.
McCoist was prompted to defend his players on Friday in the wake of club chief executive Charles Green labelling the current squad the worst in Rangers history.
The manager said his troops had done a "fantastic" job in difficult circumstances this season, but his tone was different 24 hours later.
He said: "It was a really, really poor 90 minutes. The players know what is expected of them at this club and they dropped below those standards.
"I didn't have harsh words with them in the dressing room, I just told them that it was unacceptable."
McCoist was counting on a second-half improvement similar to last week when they trailed at half-time against East Stirling, but it did not materialise.
He said: "The first half looked similar to last week and as we came in at half-time I hoped we would get a similar reaction to last week. But it was not the case and we lost two poor goals.
"You have days like this at a club like ours but sometimes it is just a poor result, but our level of performance was more disappointing than the result.
"If some players thought it was a case of turning up and getting a SFL Third Division medal then that is certainly not the case. We will sit down on Monday morning and look at it together and give the players the jolt they maybe need."