My suggestion...
(first off, I admit I know very little about the seige system and I am perfectly willing to be shot down in flames over this)
Guy one commits a crime and retreats to HV.
Guy two collects scents from crime, tracks criminal to HV and clicks 'Accusation' on wall using the scent, which then is destroyed, and thereby disables movement of hearthfires and use of crossroads for say a 30x30 radius within that wall for a period of 48hrs
Scents cannot be used again for this purpose within that 48hr period.
Wall will be marked as 'locked' and timer shown
It would still take camping of a ram, but a commitment could be made by larger factions to maybe take shifts etc for this purpose, which would also mean that this mechanic would only be used when a deal of evidence was available and the problem was large.
The 48hrs allows for gathering of forces for attack/defense
Yes the criminals would just build bigger vaults etc, but sites for these are less available, and do take more effort to maintain and monitor
edit,, another option would be to just lock the porting and HV of the criminal scented...
Insert flame below