Weaknesses
All symbiotes seem to have a weakness against very large vibrations and heat.
They are also weakened by magnesium. On Earth-616, symbiotes are vulnerable to the heat produced by high voltage electricity.
The Symbiote attached to the Edward Brock of Earth-2149 was shown to be dying after the zombification of its host; suggesting it cannot subsist upon dead flesh or tissue. Though this issue has never been addressed by its mainstream counterpart, there is nothing to suggest it could not also be the case.
Iron Man and Spider-Man (In separate universes) managed to create an antidote formula that could completely destroy a symbiote. Also, when a symbiote bonded with Wolverine, it was pushed out by his advanced healing factor, although Wolverine's healing factor had no use when he was bonded to a clone of the Venom symbiote.
On at least one occasion, Spider-Man was able to exhaust the symbiote by taking advantage of the fact that it made its webbing out of itself; after the symbiote had already used a great deal of webbing to bind him to a bell, Spider-Man forced the symbiote to use further webbing so that it would exhaust itself like blood dripping from a wound.
Since Tel-Kar was the very first wearer of the Venom symbiote, the symbiote's sync on any host whose unpredictable and has more of negative side on their personality as Tel-Kar and most of super-villains causing the symbiote went more-insane and aggressive. To cure this problem is to return to its home planet on Klyntar, like when Guardians of the Galaxy were brought by Venom-possessed Drax to that planet.