It has nothing to do with TortoiseGit but rather with Git itself.
Go to the repository's directory in your shell and execute a command
git config --add --local merge.ff true
to force only fast-forward merges for that particular repository.
Note that TortoiseGit does not include any Git substitute, — instead, it piggybacks on Git for Windows installation, so if you have TortoiseGit installed you have Git for Windows installed as well.