You can't really, but you can stop dating them and just flirt with and date someone else. From what I've seen the other person doesn't really react. I'm "dating" 3 people right now.
Britney Spears: American Dream - Level 18
Dabney Linn - STAR ID: XD2D2JVQ www.britneyspearsamericandream.tumblr.com A-List #1 - 100.1M Fans
259,638,027 Best Single's Sales
Dark Owl Records
You could just delete him from your contacts. If you go into your contacts by clicking the phone icon at the bottom right corner, and click on someone you can delete them. This is a permanent solution though, so if you might change your mind I'd suggest just dating someone new and ignoring the old boyfriend. I don't think the game notices or cares if you have more than one.
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Dabney Linn - STAR ID: XD2D2JVQ
www.britneyspearsamericandream.tumblr.com
A-List #1 - 100.1M Fans
259,638,027 Best Single's Sales
Dark Owl Records