As is so often the case, I believe the answer to this question is more nuanced than a simple yes or no, and it depends on the precise circumstances.
Does what the character is doing impact on the RAW requirements of a class, power, feature etc?
For example, lets say that the requirements for being a druid specifically state that they cannot use any metal items. If a PC druid tries to do so they simply lose access to all of the abilities, bonuses etc they get from being a druid as they don't meet the requirements.
This is a situation where I would definitely want to sit down with the player and discuss a way forward. There's nothing stopping their character behaving like a druid, but they clearly have a different set of expectations as to what being a druid actually means, and the fact that this set doesn't line up with RAW is the issue. Possible solutions might mean refluffing another class that better fits their expectations. The important thing in this situation though is that there is no room for manoeuvre, as they are not meeting one of the RAW requirements.
Is what the character is doing the antithesis of what is expected in the game fiction?
This is a particular issue when running games in well established settings. For example, in your game world, druids might never harm an animal – its just not what they would do. What you have to be clear on here is how the world and the NPCs in it would react if a druid did harm an animal.
The course of action here is to make sure the player is aware of the consequences of their character actions, whatever they might be. This could range from being a general laughing stock to having a lynch mob chasing them. If the player is comfortable with these consequences then there is no reason to disallow the action.
Is what the character doing just 'inconsistent'?
This is the catch-all for all other situations. Maybe you have a player who, up until now, has been playing a pacificist character, then suddenly they decide to attack someone. Maybe a quiet character suddenly decides to get all mouthy....the possibilities are endless.
This is a difficult situation, but my take is that it is their character, and they are entitled to react in whatever way they see fit. I would feel that it was within my remit as GM to ask in general terms why they are doing it (you never know, there might be a really good reason for the change of behaviour that you simply aren't aware of), but I would never, ever tell them to stop, and if a player were to attempt to do so then at my table that would be completely unacceptable.
There are a million and one reasons why people play RPGs. Some like to get into their characters, others like to kill monsters etc etc. Not everyone cares about creating a perfectly consistent character and you know, that's fine. However.....
When it impacts on the fun of other players at the table, you need to have a discussion with the player in question, individually at first, to explain the issue. Maybe their completely erratic behaviour is trampling over the agency of other players.