We do have write-in candidates / votes! You may vote for whomever you wish on your ballot. Primaries are a little different depending on whether they are "open" or "closed" in your state, but you may still write-in your choice if he/she is not listed on your ballot. In the general election, you are NOT forced to vote according to party lines. If you disagree with your party's candidate, vote for (or write-in) anyone you wish.
Do NOT feel forced into voting for someone you don't want to. This is very important! I get this question A LOT. You DO have have the option to vote for whomever you want to. Do not let anyone tell you otherwise.