Is there some way to stop MarketXLS from updating automatically?

MarketXLS functions work just like normal Excel functions, if you have a large spreadsheet with all the formulas in there, every time you do operations like sorting/filtering, etc, MarketXLS functions will automatically calculate and go to the servers to get new information. This sometimes has an impact on the usability of Excel sheets.

To solve this issue it is best to copy and paste all the Formula results as Values once you have got the data. A lot of information does not change frequently like the close price of a specific day in the past will always be the same. So, unless you want the data to be updated continuously (like maybe for real-time prices) please copy and paste the formula cells as values.

Another option is to turn Excel into Manual Mode.