"If a team has finished recruiting and has filled its roster spots, and wanted to hassle rival teams, could they signed several extra guys to keep them off the board?"
I think you're misunderstanding how it works. You can't sign guys unless you have spots. So for someone to hassle other teams, he'd have to cut players BEFORE signing 'extra' players. You can, however, remain at VH and keep others below High to prevent signing UNTIL the very last cycle of recruiting. At this point, the player will look for any qualifying VH or H offers (must have scholly offered and must have a roster spot). If there are none, then it goes to Moderate and looks for the same thing. Then Low. Then VL.