git remove merge commit from history

My Git history looks like that :

I would like to squash the purple commits into a single one. I don't want to see them ever again in my commit log.

I've tried to do a git rebase -i 1, but even though 1 is on the blue branch (cf. picture), I still see every commit on my purple branch.

How can I completely remove the purple branch (from commit log) ?