"This time, Sam means it," the source tells Us.
The couple -- who first debuted their relationship in May 2008 at the Cannes Film Festival -- got into a fight over the weekend at a party Samantha threw for her fashion-designer sister Charlotte Ronson at West Hollywood's Bar Marmont.
"[Sam] told security to look out for Lindsay and block her from getting in," a source tells Us. "Lindsay, of course...booked a room at [the hotel] Chateau and fully intended to crash the party and make a scene."
Bouncers kept the 22-year-old from entering the party, another source tells Us.
"She was heartbroken, but Sam did not want Lindsay entering and causing a scene and ruining Charlotte's night," the source says.
A source close to Lohan, though, insists it was the Ronson family who stayed in a room below her at the Chateau Marmont and "blasted[ed] music and would not let Samantha leave the room to meet Lindsay."
On her Twitter page, Lohan posted to Ronson: "PLEASE leave me ALONE. and stop staying in the room below me, you've woken me and my mother up. go to bed. keep cheating u win."
In addition to accusing Ronson of cheating on her, Lohan also slammed her for badmouthing her to People magazine.
Lohan disputes there was any drama, despite multiple sources, witnesses and her Twitter page telling Us otherwise.
"Oy vey. lol," she told Us over email when asked about the fight and the breakup. "Not true. hahahaha."
Lohan has denied split rumors in the past.
"We did not break up... Get your stories straight please," she blogged on MySpace in January. "It's really annoying to have all of your friends emailing you saying, I saw, I read, etc... Not True."
In December's Harper's Bazaar, Lohan declared her love for the celeb deejay.
"She's a wonderful person and I love her very much," she said.
Check out photos of Lindsay and Sam in happier times.