I couldn't find how to prevent it altogether, but some way around.
Make a larger single cell table,
Fix the height of its single row/ cell to "Exactly" = the size of your actual table.
Put your actual table inside this single cell table.
Now, if the person presses tab at the last cell of the inside table, the inside table will grow a row below, but that will not be visible to the person, and actually his cursor will go missing, so he will be confused or afraid that he might have broken down your design, and would eventually click outside the table and will get his cursor back. Then he will not bother doing it again.
In your actual table, you yourself create a row below the last row.
Select this new last row and make its font size 1.
Also, Select this last row and make its height "Exactly" .01 or whatever minimum you can achieve.
Then, if the person presses tab at the last Entry cell (which will be above this new extra row created by you), the table will still create a row, and keep on creating as long as keeps on pressing tabs, but those will be so tiny that the person will not notice much difference and will eventually give up disturbing your design.
For your this newly created last row, you can even split this single miniaturized row in 100 cells or as much you can achieve, so that if he presses tabs 99 times, nothing happens, and he doesn't even see his cursor moving, so he gives up, and was able to add just one row.