You can't arrest a sitting President just like you would anybody else, for the same reason you can't arrest Members of Congress on the way to a session. The importance of his official duties precludes it. (Doesn't matter that he's not actually fulfilling those duties, just that he has them.)
The DC police used to have fits over President Grant's speeding on capital streets, but they couldn't arrest him for it. (And before anybody says something stupid, I shall point out that President Grant, being a fancier of fast horses, had a number of them that could exceed the DC speed limit.)
The thing to do in the present case is either 1)impeach and convict him, thus removing him from office, or 2) vote him out, same result.
Arrest him while he’s golfing then. Congress-critters have been arrested for drunk driving, haven’t they? Presumably because they were not on the way to a Congressional session.
The whole principle of “Absolute power corrupts absolutely” comes into play here. If these people are guaranteed the ability to break the laws with impunity, then we’ll get nothing but law-breaking from them. That’s just a fact, and we’re seeing it right now. If we have no way to hold these people accountable to the law BETWEEN ELECTIONS, then we are guaranteed lawlessness.