One way would be to build a really fancy tomb, but when designating it as such only highlighting the tile with the coffin. This way the tomb will probably only be a 'grave' (depends on the quality of the coffin).
edit: hmm... on second thoughts, you may want to include a couple of extra tiles for storage of owned items.
I've had personal experience with this recently, so let me add my 2 cents:
If you want your soldiers to have nice tombs, that's okay. Just make sure you designate the tombs
before they die, else you won't get the chance. Maybe you can before they're buried, but I'm not sure.
Be careful about the quality of the tomb. I had a soldier buried in what must have been a legendary quality coffin once, because my countess began to promptly freak out about it once he got stuffed in there. Just a coffin, not even a single engraved tile next to it. Even though she had a much nicer (and higher value) tomb, she was continuously traumatized by it, and eventually started tantruming over it. I thought it was funny, so I left her to her misery. Until she killed someone with her pick, then I atom smashed her.
I tried experimenting with bedrooms to see how that worked, and she didn't seem to mind so much about the bedrooms. I gave my two top soldiers bedrooms even better than hers, and she was merely "put off" by it.
In fact, this turned out to be a way to mitigate her being traumatized by the tomb. It at least appears that nobles can only be upset about one person's bedroom, tomb, dining hall, office or whatnot at once. She was never put off about the bedroom and traumatized by the tomb at the same time. If you want to give your soldiers nice tombs, this might be a way around it.