Getting ownership of some files and folders back from other users?

Hello.

I have number of sub-users (SSH ones) and I used described process of giving them rights to appropriate folders / files (from here)

However, some of those users got ownership + group ownership of some (not all) of their files / folders, disabling any access to me. That causes a lot of problem for me, and ultimately to them as well.

Question: how to get back in control of those files / folders?

I tried something like (as primary user, vesic):

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find ./ ! -user vesic -exec chown vesic {} \;

or this (as secondary user, suske):

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find ./ -user suske -exec chown vesic {} \;

and both failed.

Thanks.