If you can change the file source data
If you're prepared to alter the original source CSV file, another option is to change the 'delimiter' in the data, so if your data is '4/11' (or 4-11) and Excel converts this to 4/11/2021 (UK or 11-4-2021 US), then changing the '/' or '-' character to something else will thwart the unwantwed Excel date conversion. Options may include:
- Tilde ('~')
- Plus ('+')
- Underscore ('_')
- Double-dash ('--')
- En-dash (Alt 150)
- Em-dash (Alt 151)
- (Some other character!)
Note: moving to Unicode or other non-ascii/ansi characters may complicate matters if the file is to be used elsewhere.
So, '4-11' converted to '4~11' with a tilde will NOT be treated as a date!
For large CSV files, this has no additional overhead (ie: extra quotes/spaces/tabs/formula constructs) and just works when the file is opened directly (ie: double-clicking the CSV to open) and avoids pre-formatting columns as text or 'importing' the CSV file as text.
A search/replace in Notepad (or similar tool) can easily convert to/from the alternative delimiter, if necessary.
Import the original data
In newer versions of Excel you can import the data (outlined in other answers). In older versions of Excel, you can install the 'Power Query' add-in. This tool can also import CSVs without conversion. Choose: Power Query tab/From file/From Text-CSV, then 'Load' to open as a table. (You can choose 'do not detect data types' from the 'data type detection' options).