Most commercial video editing packages will do this. I've done it at home using Sony Movie Studio. Time consuming and clunky as you have to manually track. Took me a couple of days to learn how to do it even after watching demos ... video editing is complex.
Obviously no good if you need to pixellate 10 people in a 5 minute clip - but a simple package and a trained user would be ample if you have occasional modest requirements.
There are more sophisticated tools which allow an element of tracking. I've not used it, but if you can get it past IT VirtualDub is free and there is a tracking plug-in available for a very modest price. NVeiler Video Filter plug-in . May be worth a look.
If you want totally free you can upload your video to YouTube and use their editing tools ' (joke) ...
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