Can I make fast forwarding be off by default in git?

Yes, there is --no-ff. You can configure merge options per branch, e.g.

git config branch.master.mergeoptions  "--no-ff"

adds the following to your $(REPO)/.git/config file:

[branch "master"]
    mergeoptions = --no-ff

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Footnote: speaking of my experience, I eventually found switching fast-forward to off was mostly helpful for git newcomers - however once the feel for workflows and concepts start to sink in you definitely want to avoid blurring your log graph with tons of pointless 'merged remote ..blarf' type commits.

Footnote 2, a decade later: the other answers below provide more modern config options, but really, you probably DO want to stay with the defaults (i.e. fast-forward whenever possible) in this day and age, because empty merge-commits really only make the history much more difficult to reason about.

Reading the thread of answers I ended up using the following two options

git config --global pull.ff only # Disallows non ff merges on pull. Overrides merge.ff when pulling
git config --global merge.ff false # even create extra merge commit when fast forward merge would be possible

Only loosely related I've also found this setting avoids trouble during pull

git config --global pull.rebase true # set up pull to rebase instead of merge

It seems there is still a pending question in the thread: How to do it globally (i.e. for all branches) ? For the records, we can use the following:

git config --add merge.ff false

...to make it apply to all branches in the current repository. To make it apply to all branches in all repositories where someone has not run it without the --global option (local settings override global) run this:

git config --global --add merge.ff false

From the documentation:

merge.ff
By default, git does not create an extra merge commit when merging a commit that is a descendant of the current commit. Instead, the tip of the current branch is fast-forwarded. When set to false, this variable tells git to create an extra merge commit in such a case (equivalent to giving the --no-ff option from the command line). When set to only, only such fast-forward merges are allowed (equivalent to giving the --ff-only option from the command line).