AutoPlay has now enabled for Lead Sources to be configured so that they can only be selectable when the correct Lead Type has been entered onto the lead already. 

To have leads restricted to a particular Lead Type, navigate to the Lead Source admin screen (user must have Lead Manager Admin Role access)

Settings > Sales Pipeline Settings > Lead Sources > Source

Now when you are in the edit source screen the user is presented with a new accordion

When a Lead Type is selected,  the source will only appear in the drop down list when the specified type is already set against the lead.

If a Lead Source has no specified types it will default to All Types, these sources will be selectable on a lead without the Type being set first.

Integrations, API and Lead Mappings are not affected by this restriction.