There are two URL customizations you can utilize to control the payment types for applicable membership levels.
Scheduled Payments option is uneditable
By adding &paymenttype=scheduledonly to the end of the membership form URL, the scheduled payment option will be opened when an applicable membership level is selected. Purchasers won't be able to deselect the scheduled payment option. Purchasers will be able to set up the recurring payment schedule that works for them.
Scheduled Payments option is editable
By adding &paymenttype=scheduled to the end of the membership form URL, the scheduled payment option will be opened when an applicable membership level is selected. Purchasers can deselect the scheduled payment option. Purchasers will be able to set up the recurring payment schedule that works for them.
Additional Information on URL Customizations: