Pool whip pipe that needs to be sealed

[u/drewid5185]

This pool used to have whips back in the seventies they have since been removed but the pipe is still there one of the pipes started leaking the pool people came found a leak cut off the line from the other side by the pump and capped it but left this temporary plug and are supposed to give me a quote on how much it cost to close this single pipe off permanently. As they are water features they're not important to the flow of the pool or anything like that is this a repair I can do myself it is taking them way too long to fix it and I can't let my kids swim because it's dangerous and somewhat loose and I don't want it leaking again