Can you freeze a HELOC? Sub-2/Master lease w/ option acquisition

[Ellis San Jose]

FACTS: SFH

FMV $650k

ARV $700K

1st $430k

2nd $ 51k (HELOC max line is $350k)

possible rent $3200-$3500

mtg payments $2,800

Seller is open to long term master lease or sub-2

Deal is not suitable for a quick flip.

Question: How can I control the property & keep the exiting HELOC in place but prevent the seller from utilizing additional credit from the HELOC?