Something that may help a bit is to use Klemheist instead of prusik knot. The Klemheist is directional - it only works when tension is going to be in one direction. Since that is the case with tarp lines, it's appropriate. I find that it backs off easier.
However, both knots grab the line according to how many wraps you use. Too many wraps and it's hard to release, too few wraps and it slips.
Even cinch buckles rely on tension to hold the straps. It takes some coaxing to release them too.
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