Can't change format of cell

Hello,

Using Excel 2013.
I have a document which consists of various combinations for padlocks in a format of
00-00-00

most come out correctly, but when a number combination imitates a date
i.e. 30-5-68
the cell will revert to 30/5/1968. I cannot change the format of the cell and have to manually go and reenter them all as text with the apostrophe at the beginning,
which when I am dealing with about 500 combinations and around 300 are imitating dates, can be a tad annoying and time consuming.
Does anybody have a solution please?