apex_application.g_excel_format = FALSE
to says not to export or the opposite one:
apex_application.g_excel_format = TRUE
to say to export.This solution may work with a standard report but it can't be used efficiently with Interactive Reports.
Just in case you didn't know, you can solve that problem easily. See this example at apex.oracle.com. There, I am using a function to format a string and use the value of the current request to see if the report is simply getting rendered on the page or exported. If the report is exported, I will not use the formatting tags and return the values as it was parsed.