I saw two threads in this forum that asked this question, with no good answer. I am posting this solution for anybody still struggling with this.

The question:
How can you prevent a cell's contents from overflowing into the next cell?

Of course, you can make the column wider or turn on text wrapping, but you might not want to. Each of those solutions can mess up the layout of your sheet. You may just want to truncate the value.

Some people have suggested putting a space in the next cell. This is unnecessary, a pain in the but, and will mess up any ISBLANK type formulas, among other things.