have someone on a stringhave (someone) on a string
to completely control someone's behaviour.
She can get him to do anything she wants - she's got him on a string.
have (someone) on a string
To have great influence over someone. If you want to have someone on a string, flattery always helps.
have someone on a string
or
keep someone on a string
If someone has you on a string or keeps you on a string, they can make you do whatever they want, because they control you completely. It's so unfair — she keeps him on a string and he's crazy about her.