Enforce vagrancy laws. Enforce trespassing laws. Arrest any individuals with outstanding warrants, who are violating parole, who are registered sex offenders, who are engaged in prostitution, who are in possession of stolen property, who are dealing in narcotics or are in possession of narcotics. Institutionalize those who have mental health issues.
Doing this will solve 90-95 percent of the problem with the problem with vagrants squatting on public property.
The City of Seattle has the resources to enforce current laws on the books and can do this TOMORROW. Doing so will take care of the vagrant problem - these people are NOT "homeless" - within a week.
Instead, the City will impose a $55 million a year levy on the city, money which will only go to personally enrich the grifters who pose as "homeless advocates". 
And they wonder why people can't afford to live in Seattle and why any native Seattleite with half a brain has already moved.