If you're seeing this in CC2017, there are only a couple of ways to get away from that particular pop up, but there's no way to turn it off from with DW's UI. Basically you need to either stop using tables for layout, or use the Live View editor.
If your page is an html email, or you need to display tabular data, switching to Live View and using the editing functions there, while different from Design View, will do the trick. To get to Live View's table editor...
1. You can double click a cell to add text. Or click the table so the blue tab and menu icon appear...

2. Click the 3 bar icon...

3. From there, right click and you will be presented with the usual options (insert column/row, etc). Click and drag to select multiple cells and right click to access the Merge Cells option.