This method will guide you to format cells to prevent cell contents from overflowing in Excel. Please do as follows:

1. Select the column/list that you will prevent cell contents from overflowing, right click and select the Format Cells from the right-clicking menu.

2. In the opening Format Cells dialog box, go to Alignment tab, and select Fill from the Horizontal drop down list. See screenshot:

3. Click the OK button. Then you will see all cell contents in selected column/list are not overflowing to next cells as above screenshot shown.

Notes:
(1) This method is effective when all the selected cells overflow with text; this ensures proper text cropping. However, if a cell has shorter text, the Fill function will repeat the content until the cell is full.
(2) You can also shrink selected cell contents to fit cells with checking the Shrink to fit option on the Alignment tab in Format Cells dialog box. See screenshot:

As you know, long cell content won’t overflow if next cell is not blank. This method will introduce to add space to all blank cells to prevent cell contents from overflowing easily.

1. Select the range that you will prevent cell contents from overflowing, and click Kutools > Insert > Fill Blank Cells.

2. In the opening Fill Blank Cells dialog box, check the Fixed value option, type a space into the Filled value box. See screenshot:

3. Click the Ok button.

Then you will see all cell contents are not overflowing in selected range. See screenshot:

Kutools for Excel's Fill Blank Cells utility can help Excel users quickly fill all blank cells in the specified range with values above, below, left, right, or with a fixed value, etc.