The most effective squelch I know of for this purpose is a syllabic squelch which detects human voice and ignores static crashes and tones. They are built for example in the CODAN radios and in my experience are very effective.
I found a rather old schematic published in the 73 Magazine from August 1982. The design was by W9MKV and W9YAN and used four ICs (2 x TL084C, 1 x CD4047, 1 x LM339) and was titled "Smart" Squelch for SSB.
There is an eBay kit (no relation to me and not tested by me) that seems to use the same principle. It is labeled "VOS (Voice Operated Squelch) Audio Muting Kit Form" and is from Canada.
Hope that helps,
73, John (VK2ETA)