[
'onjoin': true
{
"dimension": 0,
"mob": "srparasites",
"result": "deny"
}
]

There are several things wrong with tihs. First, what is onjoin: true doing outside your rule? It should go inside the {}.
Secondly "srparasites" is not a mob. That's a mod. So you use "mod"

alright ty

ive rewriten it, but now im wondering if this will despawn the parasites as well? that's what im looking for

[
{
"onjoion": true,
"dimension": 0,
"mod": "srparasites",
"result": "remove"
}
]

It's 'onjoin'. Not what you wrote.

Also why did you change "deny" to "remove"? That doesn't work

Already spawned mobs are not affected by this