The way Moodle stores and grade and what is displayed can be two different things. I think someone who has access to the code can fix it.
I looked at the ../mod/certificate/locallib.php file (always scary). In my 2.9 file around line 823 and line 940 I see ... GRADE_DISPLAY_TYPE_REAL, $decimals = 2);
Bet if you changed that to $decimals = 0, then that would do the trick. Now that is going to mean every certificate on the site will not see any decimals. I think this will just truncate the underlying number, essentially rounding it down for display purposes.
In the ideal world, you probably want to create a 4th possibility to displace a grade (letter, percentage, real points truncated to 2 decimal places, real points truncated to 0 decimal places).
Hope this helps,