It is not simple. I'll give you a way forward.
You can create a rule by OBBH using the Tcode FB75 (condition) and defining a constant (in replacement) that will fill out the field BSEG-ZLSPR when the rule condition is met (SYST-Tcode = 'FB75' and / or other conditions you want to insert.) So you create the block, noting that the constant chosen must be previously set as a valid reason for blocking (must exist in table T008).
To release, you must also create a rule that is more complicated, and create a set by the GS01 with usernames autoprizados to unlock.
In rule you must set the TSYST-Tcode (the transaction, usually FB02), and users (SYST-UNAM), which may allow (sure that they're on set that you created GS01).
((BKPF-Tcode = 'FB60')
OR (BKPF-Blart = 'KA')) AND (BKPF-BUKRS = 'SAP1'))
ie, if the transaction FB60 or type of document and business KA SAP1 he applies the condition set in replacement (in this example, constant = "X")
(SYST-UNAME IN USER2) AND (SYST-Tcode = 'FB02' OR SYST-Tcode = 'FBL1N') AND (BSEG-ZLSPR = 'X')
That is, if the user is on the set of GS01 (Table USER2), if the transaction FB02 or FBL1N and if the blockade is "X", it applies the condition set in substitution (constant = "" - empty, white) Which means authorized for payment.
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