Can I disable multi-tab browsing and/or screen pinching?

We're trying to lock down some iPads that are used for filling out waivers at our gym. We have them encased in acrylic cases that won't allow people to press the home or power buttons, and we're using Guided Access to lock people out of changing the URL in Safari. However, because people can pinch the screen to select other tabs, they can close the main tab (our waiver). I've deleted all the other favorites (particularly the search engines), but is there a way to completely disable this ability? If it's in the settings, I've comletely missed it.