string sb along

string sb along (deceive someone for a long time by making them believe something that is false, especially that you will help them, that you love them, etc.) — морочить голову, водить за нос

Example 1: He’s just stringing you along – he won’t marry you.

Example 2: An ironclad FBI rule – no one enters a hostage situation, ever, for no reason, period. But we are out of ideas, Molly, running on empty. He's just been stringing us along, buying time. I don't think he ever had the slightest intention of letting any of those kids go. I think he's got other plans for them. (Mary Willis Walker, "Under the Beetle's Cellar", 1995)

see also
[водить за нос]