Thank you for your question
My name is Clare
I shall do my best to help you but I need some further information first
How long have you been married and are there any children involved?
How did she access the property and what happened to the matrimonial home?
How far have the divorce proceedings reached?
I am afraid that whilst you remain married (as you are) and she has a key (which she does) then she can move in and it is in fact a (not "the") matrimonial home - and she has a financial claim on it
Unless she moves out voluntarily then I am afraid that the only way of forcing her out is to reach a financial settlement that confirms that she must leave the property.
I am sorry not to give better news but I can only explain the legal position that you are in
If you need further details please do not hesitate to ask