>>> What can people do to curb this problem?
Its very telling that the police force themselves aren't taking any disciplinary or other corrective action on the officers that keep doing this even though they have been told not to.
Conseuqently as the local police chief apparently can't/won't control their own officers to follow the law, the only practical option is to keep legally filming them and then suieing them for everything you can throw at them if they stop you and especially if they illegally mess with and damage your property (i.e. delete files from your phone). That way the economic impact alone will eventually force them to have to change.
I believe but am not 100% sure that you also have every legal right to demand to see a search warrant before you hand your phone over, but of course if you go against a cop in the street, they will probably make your day at least very inconvenient, even if you are actually right.