Spices for stuffed capsicums for 4 people as I can't use any salt and want them to be extra tasty for my Dad.

Smoked paprika would be a fabulous choice here. It's full of flavor and of course, being made from capsicums, goes perfectly. Make sure to add some vinegar or a vinegar-based hot sauce to whatever filling you use. It will brighten the dish up, as drbabs has suggested, making it wonderfully tasty. I find that sherry vinegar goes particularly well with smoked paprika. ;o)

I would sauté minced onions, garlic and mushrooms (and/ or celery, carrots, etc….endless possibilities depending on what he likes) till really well caramelized, and mix that with the beef and some fresh or dried herbs and spices for different flavor profiles. You could use thyme, oregano, a little cayenne, and some low sodium Worcestershire sauce for a Cajun take; basil, oregano, parsley with homemade tomato sauce for a sort of Italian version; thyme, cumin, sesame seeds and sumac for a salt free version of za'atar. Using lots of herbs and spices, and amping up the flavors with some acid and/or heat will help him not miss the salt.

Here's a recipe for stuffed peppers without salt and using cumin, chili powder, black pepper, coriander seed and pico de gallo or salsa.
http://blog.webicurean.com/2013/01/27/heart-healthy-stuffed-peppers/
If the spicing is too much, there are other stuffed pepper healthy recipes with less spicing.
All of them, however, seem to use ground turkey as the meat. But you could just replace that with ground beef and go ahead.