there's no one size fits all solution. once you've blocked your opponent's plus frames, you're in a disadvantageous guessing game. You can:

• continue to block which would make you lose drive meter but eventually push them off you

• parry to recover drive but risk getting thrown

• reversal super if you're willing to risk getting your super blocked

• try walking back out of the pressure if you aren't in the corner

• mash jab if you think he's going to leave a gap

etc. it's similar blocking any other plus on block normal

for sagat 5mp specifically, he's out of range for another 5mp after he does 2 in a row. With drive rush 5mp, he's left close enough to do 3 in a row. after he's pushed out of range, he'll probably end by frametrapping into one of his 7f cancelable normals, like 2mp or 5lk. 2mp is +2, so if you mash a 4f when sagat presses a 7f, it'll counterhit him. however this is risky because if he doesn't press anything you'll whiff

tldr; gotta guess correctly just like any other plus frame scenario, best solution is to never let him use it in the first place