If anyone else is wondering, you can press S (or other movement keys) to cancel arresting a suspect.

I don't know why but it is not listed under controls and I asked myself for hours how to do that.

Hint: You can use this to force a suspect to drop all weapons (otherwise some suspects keep, e.g., their pistols until animation is complete) which can be useful for the Sleeper Agent achievement in the Dark Waters DLC.