Juni,
This is actually Excel problem. You are closing and opening reference to Excel so fast, that it doesn't have time to close itself in between.
I modified your VI, first to check for errors in the main two SubVIs. Then, I edited the Open Excel and Make Visible VI, so it opens a new instance of Excel every time. This will allow the previous instance of Excel, the one when you wrote to Excel, to close itself when it is ready.
Also, monitor the CPU usage when you run this VI. I jumps way high. This could also be a problem for Excel. You may want to modify your VI or maybe add some delays to prevent the memory leaks.
I hope this works for you.
Zvezdana S.
National Instruments