For seller, there is 2 remedies i remember that is available to you, first is to enforce the original contract and second have it rescinded.
The oral agreement of sale between the two of you is binding because there is already an execution of the contract, regardless whether or not, she signed the written contract, you cannot just evict her.
My advice to you, is to make a demand letter first, ask for payment, make a legal threat, etc., make a copy and have it signed as proof of receipt. It can be useful, later on if you decide to sue in Court.
If the other party does not pay, opt for rescission of contract, using the Maceda Law, sec. 4,
you have to send him notice for the purpose of cancelling the sale, under such provisions it specifically provide for notice to party or cancellation by notarial act. What I understand about that Sec. 4, is that if there is a stipulation in the contract between the parties for rescission, notice of cancellation is enough. However, in your case, since there is no written contract, you must demand for rescission of the sale by notarial act.
I believe you have to follow the Maceda Law, it was enacted to protect buyer of house and lot by installment. After following it, you can now evict her. File Unlawful Detainer.