Back in 1981, Ronald Reagan used his executive power to order striking air-traffic controllers back to to work, after negotiations between their union, PATCO (Professional Air-Traffic Controllers), and the Federal government fell through. Those controllers who refused to return to work after 48 hours were fired and blacklisted. After a few months of headaches — strikes, airport slowdowns, etc — the strike ended and the union dissolved entirely. The blacklists were lifted in the '90s by Clinton, and air-traffic controllers eventually formed a new union to represent them, but I think the point is clear.
Police unions generally negotiate with state or local governments, not the Federal government, but the same principle applies. If a president were to write an executive order that (say) restricts the use of certain provisions in police contracts, then local governments and police unions would have to renegotiate contracts or be in open defiance of the order. At very least, that would create grounds for lawsuits, and while the president does not have to power to hire or fire non-federal law enforcement personnel, he does have executive power over Federal funds and resources that local police departments rely on, which could be restricted in districts that fail to comply with the executive order. It would be a contentious move, but not without precedent.
It's obviously unlikely that Trump would write such an executive order — even on the most generous reading Trump favors law enforcement and opposes Federal oversight — but the tactic is not unthinkable for a president who would be inclined that way.