Would Morpheus have had to force Neo to swallow the blue pill if Neo had refused the both of them?
Since it doesn't happen in the Matrix (or any related media), we can only speculate. The choices are relatively simple and we can assign a probability to each of them though.
- They just let you walk out the door.
You've likely seen at least one safe-house and may have encountered vehicles used by the Zionese. You represent a distinct security risk for the entire time that you're with the redpills. This option seems vanishingly unlikely.
- They hold you down and force-feed you a red pill.
Morpheus is very clear that taking the red pill must be (in his opinion) a conscious choice. On top of this, we know that attempting to wake someone who's unwilling tends to lead to their deaths. This option seems very unlikely
- They kill you.
The Zionese don't tend to kill without good cause, but nor do they have a problem with killing bluepills when it serves the greater cause. This option seems moderately unlikely
- They hold you down and force-feed you a blue pill.
The blue pill appears to be a common sedative. It may also have some impact on memory. This option seems to be the safest one for the Zionese and is probably the most likely.
Morpheus states clearly that he expects a choice:
"You take the blue pill, the story ends, you wake up in your bed and believe whatever you want to believe. You take the red pill, you stay in wonderland and I'll show you how deep the rabbit-hole goes."
I assume if Neo would have refused to choose, Morpheus would have suggested to take the blue pill and continue his accustomed life. That would have probably included that he forgets they ever met. I don't see why they would have killed Neo. Even if Neo insisted on not picking any pill, I think they would have given him some time, maybe a day or two, to consider his decision.
Ultimately I think as most likely option, they would have forced him to take the blue pill.
Actually, this would be the ultimate choice - take none them.
From a philosophical point of view you might say by swallowing the red pill Neo replaces a constructed world (the matrix) with an other one.
So, the ground breaking insight at this point would be that Neo can not escape the matrix no matter which pill he takes.