Excel cells hold the value Null as default. If you type something into a excel cell and then delete one might think it would return to null, but instead its now a 0-length string. (At least thats the conclusion I have arrived at)
If its a one time thing, just import the sheet to another table, then run an append query afterwards.
I had a situation where I would often import the same columns from excel sheets, but the user had a nasty habit of using the "extra/blank" columns for his own notes and such. I created some code to (invisible to user) copy the excel sheet to a new file, open it, delete Columns E-ZZ, import it, then delete the copy.
You could try a similar approach, if its something you need to do often.