I don't think you can delete days. When the reports function makes an average, it totals up for the time period and then divides by the number of days. What you want is for it to divide by the number of days you specify and I can't see where there's an option to do that. All I can think is that you should narrow the report period down to one that you trust - one that has complete food logs.
If you want a trusted average, you have to have trusted input. Picking a few days to average isn't going to tell you much, is it?
On the other hand, if it's something you really want to do exactly that way, you can always do it by hand. Pick the days, write down the amounts of calories, fat grams, etc. for each day, then divide by the number of days. At least you'll get the numbers you want. You won't get the nice graphics, bar charts, and such. Can't have everything!