It seems that the jury is still out on whether there is an effective and ethical way to prevent PTSD.
Donovan (2010) states that there isn't enough strong evidence on its efficacy. Another point it makes (on page 11) is that...
Using any beta-blocker prior to combat to prevent PTSD in military personnel should not be considered, as it would likely be more detrimental than beneficial. First, the beta-adrenergic signaling system is mediated by the catecholamines, epinephrine and norepinephrine. These hormones are central to the fight-or-flight response, and they trigger the heightened awareness necessary for soldiers to survive in combat situations.
This can be the same for others who repeatedly face violent situations.
Plus there are strong arguments for preventing any interventions which would affect the memories of those who suffer traumatic events in this way as...
Our memory preserves us and who we are and where we have been, furnishes our sense of self, and is a combination of happy moments and shameful acts.To be ourselves we cannot abandon or forget who we once were: “To alter or numb our remembrance of things past cuts to the heart of what it means to remember in a human way“.
The only ethically safe way to prevent PTSD seems to be to prevent war and all other traumatic events
Donovan, E. (2010). Propranolol use in the prevention and treatment of posttraumatic stress disorder in military veterans: forgetting therapy revisited. Perspectives in biology and medicine, 53(1), 61-74. DOI: 10.1353/pbm.0.0140 PMID: 20173296