Crystal Reports shows magnifier cursor in group fields, which hide the header section. So I don't want to use this magnifier. So how can I disable this magnifier cursor?
Short answer: it can't be done.
If you mark the Details section as 'Suppress (No Drill-Down)', the magnifying glass will still be displayed. Moreover, double-click a group-title or summary field will still generate a drill-down page, but the Details section will be suppressed (no visible).
Most users find the ability to see ('drill down') the the details as a valuable feature. You should design your report with that in mind.