I see this sentence online. "A video game reviewer has had it with threats of sexual violence contained in online messages from young boys, so she's decided to do something about it." And I cannot figure out the meaning of the first clause.
The key part of the phrase is "had it", in the sense of 'had enough of [something]' It is usually followed by 'with'.
Literally, 'had enough' would suggest that you have precisely sufficient of whatever it was you wanted - but in practise it signifies you actually have more than enough & you are not happy about it. It is probably something you never wanted at all.
Your patience is at an end. You will no longer tolerate this.
You have 'had enough of it' or more simply, 'had it' with this [something].
It is more likely to refer to behaviour of someone/thing else rather than, for instance, ice cream.
You can have had enough ice cream, but you'd be unlikely to have had it with ice cream1 - unless you never wanted to see any ever again.
Someone is upsetting you, sending threats, then you've had it with this behaviour & are going to do something about it.
1 This is, of course, completely different in meaning from
"Do you like apple pie?""
"Yes, it's lovely, I've had it with ice cream"
It is a common way of saying "having enough of something/someone" or "not willing to tolerate something/someone anymore".
Re-phrasing your example:
A video game reviewer is not willing to tolerate threats of sexual violence contained in online messages from young boys anymore, so she's decided to do something about it.