I've never seen a properly drafted term sheet that doesn't make investment contingent on review and acceptance of the formal documents, solely at the option of the investor. I've never seen a VC or institutional term sheet which didn't have a long list of conditions precedent to close such that completing the deal is effectively at the option of the investor.
If you are dealing with angels, remember that lots of things happen and there may be good reasons someone can't or chooses not to move forward.
More to the point, think about what you are asking. Why would you want to force someone to make an investment they no longer wish to complete? If you do, you better make sure your investor documents are properly drafted and that you've done full and complete disclosure. You should be prepared for continuing demands that you comply scrupulously with all of your ongoing legal and contractual obligations to your investors, minimal cooperation any time you need their assistance or consent and, most damaging, an unhappy shareholder who may let the investor community know how you've behaved. Leave the legalities aside, I don't understand why you think there is upside in pursuing this course. At a practical level you are likely to be poisoning the well from which you drink.