Depends where it is I suppose; if it's at an airport in class D airspace, it could well be illegal. If a guy is IMC and IFR, they could well go below safety altitude and lose terrain clearance thereby 'knowingly or recklessly endangering the safety of an aircraft'. Likewise if it's over a built up area, unless they're taking off and landing iaw normal aviation practice (which they won't be as they are not intending to land), they break the law if they go below 1000ft above the highest fixed object etc.
At my airfield, we've had complaints about aircraft landing outside the permitted hours. Well we know they aren't, and the aerodrome authority believe it may be people doing what you say, so the ILS is now switched off when the airfield closes.