The midfielder was snapped up from Dagenham in January for an undisclosed fee on a deal until the end of the season, which included the option of an extra 12 months.
Paul Sturrock brought Spillane to Roots Hall and started him nine times before being given the boot at the end of March, with successor Brown subsequently not handing the 24-year-old even a minute of match action before announcing his release at the end of the season.
"I didn't play for a single minute under the new manager and don't feel I was really given much of a chance under him," Spillane told the Echo.
"That's football though and there are no hard feelings from me because it's up to the manager who he picks and who he doesn't.
"But because I didn't play it wasn't a surprise that I got released, however it was obviously still disappointing."