How to stop health regen in Fallout 4?

From your clarification above, it sounds like you possibly have either:

Ghoulish perk - Radiation Damage restores lost Hit Points. In level 4, radiation level will reduce slowly.

Solar powered perk - Increased STR and END during daytime: Direct sunlight heals Radiation Damage and regenerates Hit Points.

There is a bug in Fallout 4 survival mode, where when you go to sleep while affected by an active effect from a consumable (other than satisfying thirst/hunger), that effect stays indefinitely after you wake up. You will be notified that the consumable has worn off and the effect will not be shown as "active effects" in the Pip-boy. However the effect itself can be seen in the Pip-boy.

For example, I once had a permanent 25 radiation resistance after consuming vegetable soup, with no detectable source (perks, gear, active effects). I don't know if this bug still exists (if it was patched or not) or how consistently it works and with which consumable it works. Health regen is for example granted by deathclaw omelettes.