I'm assuming by "pope" you mean anti-pope or prince-archbishop. Even if it's the actual Pope that you've managed to vassalise, the options will be similar.
If you have free investiture, you can nominate successors. However, there's no way to get the title this way. You can't nominate an heir or feudal character and whoever you nominate will become a theocracy.
Simply revoking the title is not as bad an option as you'd think. The tyranny penalty is not insurmountable (I believe it'll be -20). If you've ruled a reasonable amount of time and your realm is somewhat stable, you can do this and accept people being a bit upset with you. Hold a feast, tournament, or anything that'll raise realm opinion first.
If you want to avoid the tyranny, you need to find a reason to arrest this turbulent priest. The intrigue focus can find (or fabricate) reasons for his arrest with the Spy On decision. Antagonising him may also work- it'll make him more likely to plot against you, which would give you grounds for arrest.
When you arrest him, do everything you can to lower the chance of success. You want to fail here. This will cause him to rebel- defeat him and you can revoke his title with no complaints.
I do think it's worth noting, however, that this situation is not necessarily a bad one. A powerful theocratic vassal can be a nuisance if they like the pope more than you, but has its benefits. With Papal investiture, they receive bonuses to their cardinal eligibility, which will give you influence with the Papacy (especially if they become pope). With free investiture, they can be used for coronations, earning you a decent bonus without having to meet the Pope's demands.