Problem:

We want to ensure that only text may be entered into the cells of column A.

Solution:

Use Data Validation:
1. Select Column A.
2. Data  Data Validation  Settings  Custom
3. Enter the following formula in the Formula box:
=ISTEXT(A1)
4. Click OK.

Thus, if an entry is not text, an error message will pop up, forcing the user to try again.