It's not called a "crush" for no reason. Unrequited love is extremely painful for the giver. It can be tough for those on the receiving end as well by putting stress and upset on the mind and body. Here's a simple method to dealing with that unwanted crush.
Talk to someone about their actions when you think they have a crush on you. If someone sends you flowers, tries to hold your hand, or does anything that lets you know they have feelings for you, talk to them about it. This can get things out in the open and alleviate anxieties for both of you.
Be firm in telling them that you do not have feelings for them when an unwanted crush confesses their feelings. Make sure to note that you are very flattered, but don't give false hope for a moment.
Let it be known if something is making you uncomfortable. If they continue to pursue you, announce it loud and clear to them that it should stop. Tell them again that you do not have feelings and specify that this sort of thing makes you uncomfortable and unhappy. That usually puts the brakes on further attempts.
Make it a point to avoid spending time with your crush. If it's a work crush, go to a different place on your lunch break. If you cannot avoid them in a class, don't sit nearby. This should be done delicately. You don't want to throw it in their face that you are ignoring them.
Introduce your crush to someone who you think would be a great fit for them. This must be done carefully in order to avoid seeming like a sympathy set-up. If you do know someone who would be a great match with them, find a way to introduce them socially and casually. If it's someone who gives you the creeps, this is obviously not an option. It's great for most friendship crushes because there is no better way of getting over someone than meeting someone new.
Enlist the help of a friend if the situation continues or worsens. While you want to keep things simple, getting someone else involved will show that you are serious and not interested in playing games. Ask someone to tell them to leave you alone if they don't get the hint.
Be kind and empathetic to their situation. Let them know how sorry you truly are, but reaffirm that you do not have feelings for them.
Be honest and upfront to avoid leading someone on. Continue dating and let them know that you are seeing others if they ask.
Don't give hints. Be completely open and honest. Don't be rude in your dismissal. You can be diplomatic about saying you don't have feelings for someone. Don't spend a lot of time with someone who has a crush on you if you don't feel the same way. Even if it's your best friend, give it some time. You can go back to friendship after their crush has subsided. Time heals most crushes. Don't accept phone calls after you've told someone that you don't want them calling you any more. It is disrespectful of them to be calling you and talking to them when they do only gives them encouragement and hope for your love.