So I imported some data and ended up with a leading space in all the supposedly empty cells in a column. To remove, I needed to select the column then:

Edit > Find & Replace

Search For:  ^ * (i.e. a caret, a space, an asterisk)

Replace: leave blank

Click on Other Options and make sure:

• Current selection only
• Regular expressions

were ticked. Then Close and Replace All

https://ask.libreoffice.org/en/question/72015/how-to-remove-spaces-in-calc/