1. Set the text type to TT_MICR_CMC7 using IRecognizerParams::TextTypes. 
2. CMC-7 contains only digits and A, B, C, D and E characters, so setting a "stricter" recognition language than English may prove beneficial. To exclude all characters not used in CMC-7, set the recognition language to CMC7.

C# sample:

// Create document
FREngine.FRDocument document = engineLoader.Engine.CreateFRDocument();
// Add image file to document
//... 
//Set page processing params
FREngine.PageProcessingParams p = engineLoader.Engine.CreatePageProcessingParams();
p.RecognizerParams.SetPredefinedTextLanguage("CMC7"); //specify the language
p.RecognizerParams.TextTypes = (int)FREngine.TextTypeEnum.TT_MICR_CMC7; //specify the text type     

//Recognize document
document.Process(p, null, null);