There is a good answer here on how to create a ClearCase branch using GUI operation only. I have created such a branch and worked in it and now I wish to delete since it has been merged. I have not dared trying to simply delete the BranchType in ClearTeam Explorer since I am afraid it might corrupt something.
How to delete a branch in ClearCase and ClearTeam Explorer for Eclipse using GUI operation only?
Generally, I wouldn't recommend to delete anything, especially with ClearCase UCM.
The usual solution is to make the stream (UCM) or the brtype (base ClearCase) obsolete.
That way, the branch becomes invisible.