Escort your BF Dad to the county office to acquire the paperwork to evict. Then everyone else in the house, as a united group, meets with deadbeat, serves the eviction paperwork and tells him, in no uncertain terms, to be out by…specified date.
Prior to the set date, acquire boxes etc. On or after the date, when he leaves the house, and together as a unit, get the locksmith in(you could arrange something in advance I expect!), and change the locks.
Pack everything he owns into the boxes, put them in the garage. If he turns up and won’t leave when asked, call the police so they can witness you (again, as a united group!), giving him access to his things. Never trust your BF Dad to accomplish anything alone, he can’t do it, don’t expect him to. Plus, always have a witness in your dealing with the basement dweller. Always.
Depending on your county it may be 30, 60, or 90 days, but that time is going to pass anyway, it’s just a case of filling out the paperwork. (Way cheaper than a lawyer or an impounded car, how will he secure his cars if the guy has keys? You need to think about things like this!) Then wait for the time to pass.
Most importantly your BF Dad isn’t going to change his stripes into a guy with a spine. Do not let him drag you into his idiocy. But maybe offer to be his spine, hold his hand through each step, be at his side. You should make it clear though, if he fails to deliver or makes excuses, you’re out.
Goading a drug user is the very height of stupidity. Please, be smarter than you BF Dad!