Sometimes the "stop a ped" feature does not work. A good workaround is to use Albo1125's Arrest Manager mod: ctrl+H brings up the "scene management" tools.
I highly suggest the whole Albo1125 plugins suite (Assorted Callouts, Arrest Manager, LSPDFR+, Traffic Policer), it's more than a must have for LSPDFR: smarter AI for pursuits, a lot of indispensable tools and so on.