Hello, Martijntje,
To fix the issue correctly without any “bad coding” implementations one of our developers already posted a suggestion to WooCommerce plugin so that they could provide more possibilities for fixing this issue from our theme and also from many other themes and plugins that provide such plus/minus quantity feature.
Topic -> https://github.com/woocommerce/woocommerce/issues/37661
For this time we recommend you use next custom CSS ->
.sold-individually .quantity {
display: none;
}
You may also leave your comments or voices for this request to speed up WooCommerce devs.
Thank you for understanding!
Kind Regards,
8theme team