I’d argue the President should not be allowed to issue pardons that are:
(1) Preëmptive (i.e. absent conviction);
(2) To himself, his current or former Cabinet members, or to any of the foregoing’s current or former spouses or children or grandchildren (or their spouses); or
(3) Issued after the presidential election in the final year of their term.
Furthermore, pardons for violent offences or corruption should be prohibited; provided, however, the President should retain the power to commute such sentences, and the Congress should have the power to regulate the manner in which the President may commute such sentences.
(Notably, I don’t believe this would apply to Ulbricht. He wasn’t convicted of a violent crime.)
The Cabinet or the lame-duck pardons?
Cabinet members are close to the President and in commanding positions of authority; if they’re scared of a law they should work to change it.
Lame ducks, on the other hand, aren’t subject to the single veneer of a check on Presidential pardons: popular outrage. Limiting it in that span, when a President is unaccountable, and where we have ample history of silliness, seems warranted.
Note that I’m not proposing restricting commutations in any of those cases. (I suppose we should add a clause prohibiting the President from preëmptive commutations, too.)
> amending the constitution is virtually impossible
Not true. We’re probably closer to the end of our Constitutional stasis than at any time in our lives.
Hell, you might be able to ram something like this through today if you added a clause that nullifies past pardons per those standards.