I think yes

 please check out Professor Anke Ehkers at Oxford who developed a PTSD therapy based on memory 

Dear Christine, 

These papers may interest you: 

Article Best Practice Guide for the Treatment of Nightmare Disorder in Adults

Article Nightmares: Under-Reported, Undetected, and Therefore Untreated

Article Aggression in Nightmares and Unpleasant Dreams and in People...

Article Cognitive Phenomenology of Religious Experience in Religious...

CBT is an effective therapeutic method that can help to change thought patterns. But to actively "delete" (not suppressing) bad memories is largely impossible. Because our understanding of how the memory works at a neural level is still limited. Maybe you can look at therapeutic methods involves in dealing with bad memories that transform them to a more positive acceptable aspect. Hope this helps.

While most current therapeutic approaches seek to reappraise existing memories or forming new memories that are associated with safety, etc., there is promising research on actually deleting aversive memories, e.g., conditioned fear. The latter approach exploits the fact that, once a memory trace is activated, the reconsolidation phase presents a time window where memories can be updated or even deleted. 

See for instance: 

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3640262/

http://science.sciencemag.org/content/337/6101/1550

It seems there are some possibilities to do so;

http://news.discovery.com/human/erasing-and-restoring-the-brains-memories-140601.htm

Anyway, as you all say, CBT as it is used for PTSD is of course of value.

But Christine, sometimes is not necessary some invasive method or any kind of intervention to change memories, this TED talk is interesting on how memories can be shaped by circumstances. That is what CBT addresses, in this case it is not limited to people with any disorder as PTSD but people in general.

http://www.ted.com/talks/elizabeth_loftus_the_fiction_of_memory

Dear Christine,

I have found the phenomenon of autogenic abreaction and autogenic neutralization to be powerful methods of addressing disturbing memories. To begin with, you may wish to read my paper on the treatment of recalcitrant nightmares with autogenic abreaction. I also addressed the entire process in one of my books and I will be happy to send you a chapter or two if you are interested. I am attaching a copy of the paper I referred to earlier.

With best wishes,

Micah

Dear Christine,

If you ask whether it is possible to actively suppress bad memories the answer is yes. I think you might be interested by the « Think/No-Think » paradigm (TNT) developed by Michael Anderson (University of Cambridge), see especially the review by Anderson & Huddleston (2011). The core idea of this paradigm is that it is possible to actively forget specific representations if you prevent those memories to enter in your mind. Exerting an inhibitory control over memories while the traces are reactivated (e.g. intrusive memories) can indeed impair the reconsolidation process on a neurobiological level or even lead to their direct suppression. 

The cognitive task developed to induce such forgetting run as follow:

After learning a set of associations between pairs of cue-target (e.g. a word and a negative picture), participants practice the Think/No-Think task where, once the cue is presented they may either think to the related picture (Think condition) or to prevent the target to enter their mind by focusing their attention on the cue (No-Think condition). Another third of the association constitute the Baseline condition. The cued recall of the associations show that items of the No-Think condition are more forgotten than the Baseline condition, itself more forgotten than the Think condition.

Since the first publication (Anderson & Green, 2001) this result has been consistently reproduced, providing light on the neural basis of the inhibitory system allowing direct suppression of memory traces in the hippocampus (e.g. Gagnepain et al., 2014). But it also appeared that the inhibitory system underpinning the “No-Think” effect can be impaired under some psychopathological conditions (see e.g. Fawcett et al. 2015 on rumination). I am especially focusing on anxiety and depression in my researches but PTSD and OCD are also concerned. TNT paradigm then show promising results for therapies based either on the improvement of inhibitory control abilities induced by TNT practice or via adaptation of the TNT with specific content (e.g. intrusive memories), both methods inducing forgetting of bad memories over the long term.

But if you ask whether it is possible to prevent the incorporation of bad memories into dream, you might also check this article (Spoormaker, 2008) with an insightful proposal for recurrent nightmare therapies based on memory and scripts.

Hope it helps.

All the best,

Nicolas

See here a recent, very good, paper on the topic. 

Positive pre sleep cognitive activity