No, Sage 50 currently has no built-in facility to force capital letters, and no spell check capability.

If the invoices are mostly inventory & service items, you can set them up and keep them maintained, then the invoice details will be in the same case as the item descriptions.   For the Inventory & Service Items, the Item field is case-sensitive, so CAT 1x1234, cat 1x1234, and Cat 1x1234 are three different item numbers.

It is annoying going through invoices and having some with capital letters and some without, especially when dealing with hundreds of invoices in the same vendor account. It makes it hard to tell if any characters are missing.

For vendor invoice numbers, all you can do is ask people to be careful, and help the ones that can't / won't, to find something more suitable to their talents.  

The Journal Entry report can be configured to show the login name of the person who entered it, if that helps.   You can show the offender where the capslock key is.