Excel formatting issue with “time”

so time entry is one of the most common issues. I'm not clear on what your specific issue is. It sounds like you should be able to highlight the whole column and select 'General' for the formatting. If that doesn't work you could try 'clear formatting'. But that said let me say a couple things about time formatting:
a) if you enter something in a format that Excel recognizes as a time and the cell is NOT set to force another format like TEXT, then excel will convert that entry into a 'time' entry. It will be displayed in the default time format (unless another is selected). So if the default time format for you is H:MM AM/PM and the cell is set to general then it won't matter if you enter 1:15 PM or 13:15 the VALUE will be the same and the DISPLAY will be 1:15 PM. But if you enter '1:15 pm it may LOOK just like 1:15 pm but is really just text because that apostrophe beforehand tells excel to treat it as text. The same is true if you import text or if the cell is set to text when you enter the value.
so back to the problem at hand, I don't see how setting that column to 24H time format is an issue. If you enter 11:15 pm it will convert to 23:15. If they enter 11:15 it will be 11:15 am in either case and if you 11:15p or 11:15pm it will be "text" in either case.
Maybe you should consider Data Validation (Data->Data Tools->Data Validation) and then select that the input must be Time and select 'greater than or equal to' (instead of 'between') and enter 0:00 for the start. You can then even customize the error message that those users will get when they enter something that excel doesn't recognize as time.  Note if you want make sure they don't enter a different day like 1/2/2024 11:00 then you can set the data validation to 'between' and use 0:00 and 23:59:59.99