How To Reset After Merging A Git PR By Mistake

By mistake I merged a git pull request (someone else has to merge it, not me...) how can I roll back and create the pull request again

this is what I did:

1) created a pull request to merge branchA into branchB (PR #44)

2) (by mistake) merged the pull request #100 (on GitHub site)

3) Clicked the Revert button to "create a new pull request to revert these changes" (PR #45)

4) Merged pull request #45

Now I am trying to again create a pull request to merge branchA into branchB but it does not let me because "There aren't any commits to merge" (I am using git desktop gui on Mac)

Should I reset branchB to the last commit before action (1) ?

And then create a pull request to merge branchA into branchB ?

If yes, how? Would this be correct?

When on branchB:

git reset --hard 4f5oo77

git commit -am "reset to the last commit before merging PR 44"

git push origin

EDIT: this is the log history on branchB

I wan to go back to commit "4f5oo77 2015-12-01 | Merge pull request #36 from my-repo/branchA"

*   87a8888 2015-12-16 | Merge pull request #45 from my-repo/revert-44-branchA (HEAD, origin/branchB, branchB)
|\  
| * 55bd11c 2015-12-16 | Revert "merge branchA" (origin/revert-44-branchA) 
|/  
*   y77447a 2015-12-16 | Merge pull request #44 from my-repo/branchA 
|\  

....

| |/  
* |   4f5oo77 2015-12-01 | Merge pull request #36 from my-repo/branchA
...

EDIT-2 This is what ended up doing:

git checkout branchB
git reset --hard 4f5oo77
git push --force-with-lease

At this point branchB was back to its "state" before the unwanted merges and I was able to create a pull request for merging branchA into branchB

Except if you REALLY can't modify an history that you already shared (pushed), I think that it's much better to use 'git reset' and 'git push --force-with-lease' instead of ditching your history by stacking revert commits!