Seller can cancel anytime, BUT there will be many penalties & potential lawsuits. Your last chance is if the buyer misses the mortgage contingency. That is about the last contingency to clear.
If you mean a sales contract, you can't typically just "cancel" it once you sign it (state law may have some sort of right of rescission; not sure how OH works or if it applies to real estate). It is binding on you (and typically your heirs and assigns) as much as on the buyers. It is usually a lot easier for buyers to back out than sellers, as buyers normally have one or more contingencies written into the sales agreement.
Of course, no one can force you to sell your house (well, a judge theoretically could if the buyer sues you for specific performance and wins, but this is rare). If you enter a valid, binding contract and fail to close (e.g., just change your mind and don't sign the closing documents), you would be in default of the agreement. The broker might then sue for the commission, and depending on the circumstances, the buyer might sue for breach of contract, etc.
If you have a legitimate reason to withdraw and the buyer is understanding, you can always terminate by mutual agreement. Buyer and seller would sign a release from the contract, and also release the hand money back to the buyer.
Listing contracts will specify how they can be terminated. Typically a listing contract gives a particular broker an exclusive right to sell a property for a set amount of time. This means the broker earns a commission if the house sells -- no matter who finds the buyer. Most can be terminated after some pre-determined number of days (e.g., 180 or 90), through a process described in the contract. In my area, it's a certified letter and 10 days' notice.
If you just want to withdraw your house from the market, most MLS's will have a mechanism for this. You can change the status to "Withdrawn" or "Inactive" or whatever the equivalent is in your area, so that it will disappear from Realtor.com and other advertising sites, and no longer be shown.
The contract would remain in force, however, and you could not sell the house by other means during the contract term. This also means you can reinstate it by simply changing the status back to Active. Some sellers temporarily withdraw a property in this manner in order to address condition issues from failed inspections, or due to holidays or other personal reasons.
Good luck in any case!