If you want to let a relationship lapse, my answer to a previous question may be valuable in creating distance and gracefully ending a relationship.
Avoid giving justification or indicating you are interested in spending time with this person in the future.
If/when this person contacts you via social media/texting/messaging channels, don't respond immediately, don't ask questions or extend/expand conversations (this applies to in person one on one conversations as well).
Relationships between people, including friendships, require effort to maintain. Unless this person has a specific desire to develop a friendship with you and is extremely persistent about it, your lack of effort or response will discourage this person from putting effort in.
The "I am too busy" excuse is a polite social standard way to decline an invitation and there is nothing wrong with using it multiple times. If you are against giving the same response each time, you could word it differently each time though, cite upcoming deadlines, new responsibilities, recent colds, using little white lies or exaggeration to add some variety to your declination.
When you get these invitations to spend time with your acquaintance and her partner, wait to respond, a day or maybe more.
The idea is to gradually ramp up the amount of effort required to reach and interact with you until it is not worth the other's time. If you are still getting invites after declining with late messages, try not responding until the day of the meeting, after the meeting date, or not at all. Very few people care to repeatedly try talking to somebody who doesn't respond.