stopping someone from joining police

[whyyyyyisitmonddayy]

ex
includes threatening behaviour, saying I can't be rid of them, have to do as they say or they'll make my life hard
harming animals / killing plants.
inappropriate / indecent exposure
clear sociopathic tendencies. very emotionally manipulative.
how do I report?

You can't stop them from joining the police. You can report them for whatever crimes you think they have committed by calling 101. It'll be up to the vetting process to decide on the information they have whether to give them the appropriate vetting they need to join the job.

If they’ve been charged with indecent exposure won’t that stop them?
how old are they?

Killing plants??

pouring bleach onto plants they were meant to be caring for as a favour to a relative

not charged - wasn't actually reported by witnesses but certainly happened, some time ago
mid / late 20s

I'll report them. he's very mentally unstable. obviously not safe

I think the country's police forces probably get many reports about people who are applying to join them, some genuine, some possibly less so.
If you have evidence to back up your report I would think you need to include it, otherwise your information may carry less weight.
Especially if you report anonymously.

Mid 20s now.. how old were you when you dated and how long ago?

Did you report these things at the time they happened? Do you have any evidence, if not it would be your word against his.

Do it because it would at least make you feel better telling them what you know.

But don't expect it to make a difference without any evidence.