Removing merge arrows and versions of files is straightforward using the ClearCase CLI. However, I only can manipulate our UCM via the Eclipse Clearteam Plugin and I do not have access to the command line.
Right now the version tree in the area I want to manipulate looks like this:

For reference, here is the toolbar that I have at the ready. The help menu only described how to make merges, not cancel them:

How do I remove a merge through the Clearteam application shown?
There is not much option that doing the equivalent of a
cleartool rmverof the target version of the merge arrow. (right-click on the target version, and select delete)And that would even not be enough, as ClearCase should refuse the
rmver, considering the version is reference by the merge hlink, or part of other UCM metadata (baselines, stream, ...)If that is the case, then the GUI alone would not be enough, as opposed to the CLI
rmver -xhlink.You might consider adding a custom action menu to your ClearTeam Explorer in order to try and replicate the
rmver -xhlinkusing the CM API (libraries and documentation are an option when you install ClearCase), but I don't know if said library can go that far.