Is it ok to masturbate but not ejeculate?

Sure: It's common for a man to enjoy himself but save the climax for later. Some sects have prohibitions on who can and can't come to group prayers depending on when he'd had an ejaculation; this seems superstitious to me but it's not my place to say. If this applies to you, try to avoid any practice that seems unhealthy or uncomfortable -- and perhaps decide it's no one else's business.